udp: Fix pkt_size management

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

Problem introduced in 66028b7ba6

Bug-Id: 835

libavformat/udp.c

@@ -464,7 +464,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
-    if (s->pkt_size)
+    if (s->pkt_size > 0)
         h->max_packet_size = s->pkt_size;
 
     p = strchr(uri, '?');