core: fix memory leak on failed preset-all

How to reproduce
$ systemctl set-default multi-user # https://github.com/systemd/systemd/issues/2298
$ systemctl preset-all
Failed to execute operation: Too many levels of symbolic links

$ systemctl poweroff

Fixes:
==1==
==1== HEAP SUMMARY:
==1==     in use at exit: 65,645 bytes in 7 blocks
==1==   total heap usage: 40,539 allocs, 40,532 frees, 30,147,547 bytes allocated
==1==
==1== 109 (24 direct, 85 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 7
==1==    at 0x4C2BBCF: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x4C2DE2F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x23DA71: unit_file_changes_add (install.c:233)
==1==    by 0x23E45D: remove_marked_symlinks_fd (install.c:453)
==1==    by 0x23E267: remove_marked_symlinks_fd (install.c:405)
==1==    by 0x23E641: remove_marked_symlinks (install.c:494)
==1==    by 0x243A91: execute_preset (install.c:2190)
==1==    by 0x244343: unit_file_preset_all (install.c:2351)
==1==    by 0x18AAA2: method_preset_all_unit_files (dbus-manager.c:1846)
==1==    by 0x1D8157: method_callbacks_run (bus-objects.c:420)
==1==    by 0x1DA9E9: object_find_and_run (bus-objects.c:1257)
==1==    by 0x1DB02B: bus_process_object (bus-objects.c:1373)
==1==
==1== LEAK SUMMARY:
==1==    definitely lost: 24 bytes in 1 blocks
==1==    indirectly lost: 85 bytes in 1 blocks
==1==      possibly lost: 0 bytes in 0 blocks
==1==    still reachable: 65,536 bytes in 5 blocks
==1==         suppressed: 0 bytes in 0 blocks
==1== Reachable blocks (those to which a pointer was found) are not shown.
==1== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==1==
==1== For counts of detected and suppressed errors, rerun with: -v
==1== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

0 files changed, 0 insertions, 0 deletions