Fix MultiplayerAPI crash when peer impl misbehave.

Also fix WebSocketMultiplayer::get_available_packet_count() return value
when peer is not configured to use the multiplayer API.

core/io/multiplayer_api.cpp

@@ -111,6 +111,7 @@ void MultiplayerAPI::poll() {
 		Error err = network_peer->get_packet(&packet, len);
 		if (err != OK) {
 			ERR_PRINT("Error getting packet!");
+			break; // Something is wrong!
 		}
 
 		rpc_sender_id = sender;