udp: don't realloc in receiving, just set i_buffer

Similar than rtp input does, doing realloc from 64k->1.5k would
usually mean extra malloc+memcpy per packet.

Also don't pass buffer onward if we don't receive any useful stuff (-1)

modules/access/udp.c

@@ -264,10 +264,10 @@ static void* ThreadRead( void *data )
             if( errno == EINTR )
                 break;
             continue;
+        } else {
+            pkt->i_buffer = len;
+            block_FifoPut( sys->fifo, pkt );
         }
-
-        pkt = block_Realloc( pkt, 0, len );
-        block_FifoPut( sys->fifo, pkt );
     }
 
     block_FifoWake( sys->fifo );