libavformat/udp: Fix call to recvfrom(2)

Last parameter to recvfrom() is not initialized, causing
recvfrom() to randomly return EINVAL. Issue was introduced in
af04a27893.

libavformat/udp.c

@@ -543,6 +543,7 @@ static void *circular_buffer_task_rx( void *_URLContext)
     }
     while(1) {
         UDPQueuedPacketHeader pkt_header;
+        pkt_header.addr_len = sizeof(pkt_header.addr);
 
         pthread_mutex_unlock(&s->mutex);
         /* Blocking operations are always cancellation points;