libvncclient: ensure WaitForMessage() says 1 in case of buffered data

Previously, WaitForMessage() would only check the socket for new data
using select(), even when ReadFromRFBServer had already buffered
unprocessed data. This could cause unnecessary timeouts and delays in
message processing.

Author: bk138

src/libvncclient/sockets.c

@@ -976,6 +976,12 @@ int WaitForMessage(rfbClient* client,unsigned int usecs)
       return 1;
     }
 
+  /* Check if we have buffered data available */
+  if (client->buffered > 0) {
+    client->serverMsg = TRUE;
+    return 1;
+  }
+
   client->serverMsg = client->serverMsgMulticast = FALSE;
 
   timeout.tv_sec=(usecs/1000000);