lavf/msnwc: Return 0 if the probe function does not detect msnwc-tcp.

Reduces console spamming on debug level.

libavformat/msnwc_tcp.c

@@ -67,7 +67,7 @@ static int msnwc_tcp_probe(AVProbeData *p)
         }
     }
 
-    return -1;
+    return 0;
 }
 
 static int msnwc_tcp_read_header(AVFormatContext *ctx)