Formspecs: Remove accidental empty 'quit' field

src/network/serverpackethandler.cpp

@@ -1511,7 +1511,8 @@ void Server::handleCommand_InventoryFields(NetworkPacket* pkt)
 	if (peer_state_iterator != m_formspec_state_data.end()) {
 		const std::string &server_formspec_name = peer_state_iterator->second;
 		if (client_formspec_name == server_formspec_name) {
-			if (fields["quit"] == "true")
+			auto it = fields.find("quit");
+			if (it != fields.end() && it->second == "true")
 				m_formspec_state_data.erase(peer_state_iterator);
 
 			m_script->on_playerReceiveFields(playersao, client_formspec_name, fields);