xcb: Warn and bail out when even the basic, dummy context fails

Do not move on to glGet since the behavior is undefined.

Task-number: QTBUG-48986
Change-Id: Ifd279635ed1b8441f92697965d15ae3ecb59a7e3
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/plugins/platforms/xcb/gl_integrations/xcb_glx/qglxintegration.cpp

@@ -672,8 +672,11 @@ void QGLXContext::queryDummyContext()
     }
 
     QOpenGLContext context;
-    context.create();
+    if (!context.create() || !context.makeCurrent(surface.data())) {
-    context.makeCurrent(surface.data());
+        qWarning("QGLXContext: Failed to create dummy context");
+        m_supportsThreading = false;
+        return;
+    }
 
     m_supportsThreading = true;