[FS9] Add missing null termination when serializing QStrings

The produced QByteArray was missing the null terminating character,
since QByteArray::append() only added up to and not including '\0'.
This was also the reason why sometimes planes did not change their
aircraft models properly.

src/plugins/simulator/fs9/multiplayerpacketparser.cpp

@@ -29,7 +29,7 @@ namespace BlackSimPlugin
 
             QByteArray writeValue(QByteArray data, const QString &value)
             {
-                data.append(qPrintable(value));
+                data.append(qPrintable(value), value.size() + 1);
                 return data;
             }