udp_server: fix crash when dialog opens for the first time (#1002)

UDP server dialogue crashes when theres no protocol stored in settings,
as the default is wrongly "JSON", but it should be "json".

This patch uses default "json" even when there is wrong protocol in
settings to prevent further crashes.

plotjuggler_plugins/DataStreamUDP/udp_server.cpp

@@ -97,9 +97,13 @@ bool UDP_Server::start(QStringList*)
 
   // load previous values
   QSettings settings;
-  QString protocol = settings.value("UDP_Server::protocol", "JSON").toString();
   QString address_str = settings.value("UDP_Server::address", "127.0.0.1").toString();
   int port = settings.value("UDP_Server::port", 9870).toInt();
+  QString protocol = settings.value("UDP_Server::protocol").toString();
+  if (parserFactories()->find(protocol) == parserFactories()->end())
+  {
+    protocol = "json";
+  }
 
   dialog.ui->lineEditAddress->setText(address_str);
   dialog.ui->lineEditPort->setText(QString::number(port));