shared: fix double free in link

Fixes:
```
touch hola.service
systemctl link $(pwd)/hola.service $(pwd)/hola.service
```

```
==1==ERROR: AddressSanitizer: attempting double-free on 0x60300002c560 in thread T0 (systemd):
    #0 0x7fc8c961cb00 in free (/lib64/libasan.so.3+0xc6b00)
    #1 0x7fc8c90ebd3b in strv_clear src/basic/strv.c:83
    #2 0x7fc8c90ebdb6 in strv_free src/basic/strv.c:89
    #3 0x55637c758c77 in strv_freep src/basic/strv.h:37
    #4 0x55637c763ba9 in method_enable_unit_files_generic src/core/dbus-manager.c:1960
    #5 0x55637c763d16 in method_link_unit_files src/core/dbus-manager.c:2001
    #6 0x7fc8c92537ec in method_callbacks_run src/libsystemd/sd-bus/bus-objects.c:418
    #7 0x7fc8c9258830 in object_find_and_run src/libsystemd/sd-bus/bus-objects.c:1255
    #8 0x7fc8c92594d7 in bus_process_object src/libsystemd/sd-bus/bus-objects.c:1371
    #9 0x7fc8c91e7553 in process_message src/libsystemd/sd-bus/sd-bus.c:2563
    #10 0x7fc8c91e78ce in process_running src/libsystemd/sd-bus/sd-bus.c:2605
    #11 0x7fc8c91e8f61 in bus_process_internal src/libsystemd/sd-bus/sd-bus.c:2837
    #12 0x7fc8c91e90d2 in sd_bus_process src/libsystemd/sd-bus/sd-bus.c:2856
    #13 0x7fc8c91ea8f9 in io_callback src/libsystemd/sd-bus/sd-bus.c:3126
    #14 0x7fc8c928333b in source_dispatch src/libsystemd/sd-event/sd-event.c:2268
    #15 0x7fc8c9285cf7 in sd_event_dispatch src/libsystemd/sd-event/sd-event.c:2627
    #16 0x7fc8c92865fa in sd_event_run src/libsystemd/sd-event/sd-event.c:2686
    #17 0x55637c6b5257 in manager_loop src/core/manager.c:2274
    #18 0x55637c6a2194 in main src/core/main.c:1920
    #19 0x7fc8c7ac7400 in __libc_start_main (/lib64/libc.so.6+0x20400)
    #20 0x55637c697339 in _start (/usr/lib/systemd/systemd+0xcd339)

0x60300002c560 is located 0 bytes inside of 19-byte region [0x60300002c560,0x60300002c573)
freed by thread T0 (systemd) here:
    #0 0x7fc8c961cb00 in free (/lib64/libasan.so.3+0xc6b00)
    #1 0x7fc8c90ee320 in strv_remove src/basic/strv.c:630
    #2 0x7fc8c90ee190 in strv_uniq src/basic/strv.c:602
    #3 0x7fc8c9180533 in unit_file_link src/shared/install.c:1996
    #4 0x55637c763b25 in method_enable_unit_files_generic src/core/dbus-manager.c:1985
    #5 0x55637c763d16 in method_link_unit_files src/core/dbus-manager.c:2001
    #6 0x7fc8c92537ec in method_callbacks_run src/libsystemd/sd-bus/bus-objects.c:418
    #7 0x7fc8c9258830 in object_find_and_run src/libsystemd/sd-bus/bus-objects.c:1255
    #8 0x7fc8c92594d7 in bus_process_object src/libsystemd/sd-bus/bus-objects.c:1371
    #9 0x7fc8c91e7553 in process_message src/libsystemd/sd-bus/sd-bus.c:2563
    #10 0x7fc8c91e78ce in process_running src/libsystemd/sd-bus/sd-bus.c:2605
    #11 0x7fc8c91e8f61 in bus_process_internal src/libsystemd/sd-bus/sd-bus.c:2837
    #12 0x7fc8c91e90d2 in sd_bus_process src/libsystemd/sd-bus/sd-bus.c:2856
    #13 0x7fc8c91ea8f9 in io_callback src/libsystemd/sd-bus/sd-bus.c:3126
    #14 0x7fc8c928333b in source_dispatch src/libsystemd/sd-event/sd-event.c:2268
    #15 0x7fc8c9285cf7 in sd_event_dispatch src/libsystemd/sd-event/sd-event.c:2627
    #16 0x7fc8c92865fa in sd_event_run src/libsystemd/sd-event/sd-event.c:2686
    #17 0x55637c6b5257 in manager_loop src/core/manager.c:2274
    #18 0x55637c6a2194 in main src/core/main.c:1920
        #19 0x7fc8c7ac7400 in __libc_start_main (/lib64/libc.so.6+0x20400)

previously allocated by thread T0 (systemd) here:
    #0 0x7fc8c95b0160 in strdup (/lib64/libasan.so.3+0x5a160)
    #1 0x7fc8c90edf32 in strv_extend src/basic/strv.c:552
    #2 0x7fc8c923ae41 in bus_message_read_strv_extend src/libsystemd/sd-bus/bus-message.c:5578
    #3 0x7fc8c923b0de in sd_bus_message_read_strv src/libsystemd/sd-bus/bus-message.c:5600
    #4 0x55637c7639d1 in method_enable_unit_files_generic src/core/dbus-manager.c:1969
    #5 0x55637c763d16 in method_link_unit_files src/core/dbus-manager.c:2001
    #6 0x7fc8c92537ec in method_callbacks_run src/libsystemd/sd-bus/bus-objects.c:418
    #7 0x7fc8c9258830 in object_find_and_run src/libsystemd/sd-bus/bus-objects.c:1255
    #8 0x7fc8c92594d7 in bus_process_object src/libsystemd/sd-bus/bus-objects.c:1371
    #9 0x7fc8c91e7553 in process_message src/libsystemd/sd-bus/sd-bus.c:2563
    #10 0x7fc8c91e78ce in process_running src/libsystemd/sd-bus/sd-bus.c:2605
    #11 0x7fc8c91e8f61 in bus_process_internal src/libsystemd/sd-bus/sd-bus.c:2837
    #12 0x7fc8c91e90d2 in sd_bus_process src/libsystemd/sd-bus/sd-bus.c:2856
    #13 0x7fc8c91ea8f9 in io_callback src/libsystemd/sd-bus/sd-bus.c:3126
    #14 0x7fc8c928333b in source_dispatch src/libsystemd/sd-event/sd-event.c:2268
    #15 0x7fc8c9285cf7 in sd_event_dispatch src/libsystemd/sd-event/sd-event.c:2627
    #16 0x7fc8c92865fa in sd_event_run src/libsystemd/sd-event/sd-event.c:2686
    #17 0x55637c6b5257 in manager_loop src/core/manager.c:2274
    #18 0x55637c6a2194 in main src/core/main.c:1920
    #19 0x7fc8c7ac7400 in __libc_start_main (/lib64/libc.so.6+0x20400)

SUMMARY: AddressSanitizer: double-free (/lib64/libasan.so.3+0xc6b00) in free
==1==ABORTING
```

Closes #5015

1 files changed, 6 insertions, 2 deletions

diff --git a/src/shared/install.c b/src/shared/install.c
index 8036b0a404..b7e8f14531 100644
--- a/src/shared/install.c
+++ b/src/shared/install.c
@@ -1941,7 +1941,7 @@ int unit_file_link(
                 unsigned *n_changes) {
 
         _cleanup_lookup_paths_free_ LookupPaths paths = {};
-        _cleanup_free_ char **todo = NULL;
+        _cleanup_strv_free_ char **todo = NULL;
         size_t n_todo = 0, n_allocated = 0;
         const char *config_path;
         char **i;
@@ -1990,7 +1990,11 @@ int unit_file_link(
                 if (!GREEDY_REALLOC0(todo, n_allocated, n_todo + 2))
                         return -ENOMEM;
 
-                todo[n_todo++] = *i;
+                todo[n_todo] = strdup(*i);
+                if (!todo[n_todo])
+                        return -ENOMEM;
+
+                n_todo++;
         }
 
         strv_uniq(todo);