udp: Fix local_port management

The default value for unset is -1, not 0.

Problem introduced in 66028b7b

libavformat/udp.c

@@ -519,7 +519,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
-    if ((s->is_multicast || !s->local_port) && (h->flags & AVIO_FLAG_READ))
+    if ((s->is_multicast || s->local_port < 0) && (h->flags & AVIO_FLAG_READ))
         s->local_port = port;
 
     if (localaddr[0])