lavf/udp: fix dead code.

Since d607861, service can not be NULL, only "0".
An UDP address with neither local port nor address leaves both
service and node to their default value, and POSIX specifies
that they are not allowed to be both NULL; "0" is equivalent
to an unspecified port for all currently known protocols.

Fix CID 1341570.

libavformat/udp.c

@@ -264,7 +264,7 @@ static struct addrinfo *udp_resolve_host(URLContext *h,
         res = NULL;
         av_log(h, AV_LOG_ERROR, "getaddrinfo(%s, %s): %s\n",
                node ? node : "unknown",
-               service ? service : "unknown",
+               service,
                gai_strerror(error));
     }