Return 0 instead of -1 on enet_protocol_receive_incoming_commands

Return 0 instead of -1 on enet_protocol_receive_incoming_commands when nothing received.
This allows the Service loop to continue running and not return an error when there is nothing to do with the socket receive.

From debugging I found sometimes the enet_protocol_receive_incoming_commands returns -1 because simply nothing happened in the 256 for loop.

Other functions like enet_protocol_send_outgoing_commands return 0 when nothing happened.

protocol.c

@@ -1288,7 +1288,7 @@ enet_protocol_receive_incoming_commands (ENetHost * host, ENetEvent * event)
        }
     }
 
-    return -1;
+    return 0;
 }
 
 static void