Fix potential crash when Qt3 is used as part of another application due to multiple calls to XLib threading init function

src/kernel/qapplication_x11.cpp

@@ -1646,7 +1646,8 @@ void qt_init_internal( int *argcptr, char **argv,
     setlocale( LC_NUMERIC, "C" );	// make sprintf()/scanf() work
 
 #if defined(QT_THREAD_SUPPORT)
-    if ( qt_is_gui_used ) {
+    if (( qt_is_gui_used ) && ( !display )) {
+	// If Qt is running standalone with a GUI, initialize X11 threading
 	XInitThreads();
     }
 #endif