resolved: don't stop handle messages after receiving a zero length UDP packet (#3323)

Fixes:

-bash-4.3# ss --udp -l -p
State      Recv-Q Send-Q Local Address:Port                 Peer Address:Port
UNCONN     0      0          *:5355                     *:* users:(("systemd-resolve",pid=601,fd=12))
UNCONN     0      0         :::5355                    :::* users:(("systemd-resolve",pid=601,fd=14))

-bash-4.3# nping --udp -p 5355 --data-length 0 -c 1 localhost

-bash-4.3# journalctl -u systemd-resolved -b --no-hostname
...
May 21 14:59:22 systemd-resolved[601]: Event source llmnr-ipv4-udp (type io) returned error, disabling: Input/output error
...

-bash-4.3# nping --udp -p 5355 --data-length 1000 -c 1 localhost

-bash-4.3# ss --udp -l
State      Recv-Q Send-Q Local Address:Port                 Peer Address:Port
UNCONN     2304   0          *:5355                     *:*
UNCONN     0      0         :::5355                    :::*

1 files changed, 2 insertions, 3 deletions

diff --git a/src/resolve/resolved-manager.c b/src/resolve/resolved-manager.c
index b3ff46b5da..9600bde1e9 100644
--- a/src/resolve/resolved-manager.c
+++ b/src/resolve/resolved-manager.c
@@ -643,6 +643,8 @@ int manager_recv(Manager *m, int fd, DnsProtocol protocol, DnsPacket **ret) {
         mh.msg_controllen = sizeof(control);
 
         l = recvmsg(fd, &mh, 0);
+        if (l == 0)
+                return 0;
         if (l < 0) {
                 if (errno == EAGAIN || errno == EINTR)
                         return 0;
@@ -650,9 +652,6 @@ int manager_recv(Manager *m, int fd, DnsProtocol protocol, DnsPacket **ret) {
                 return -errno;
         }
 
-        if (l <= 0)
-                return -EIO;
-
         assert(!(mh.msg_flags & MSG_CTRUNC));
         assert(!(mh.msg_flags & MSG_TRUNC));