udp: Receive on the remote port number, if no local port is set

For a unicast udp stream to localhost:1234, currently
ffplay udp://:1234?localport=1234
works, but
ffplay udp://:1234
doesn't work.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/udp.c

@@ -365,7 +365,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
-    if (s->is_multicast && (h->flags & AVIO_FLAG_READ))
+    if ((s->is_multicast || !s->local_port) && (h->flags & AVIO_FLAG_READ))
         s->local_port = port;
     udp_fd = udp_socket_create(s, &my_addr, &len);
     if (udp_fd < 0)