Prevent autoloading of libGQt when calling gVirtualX->InheritsFrom("T…

…GQt")

This fixes a potential crash when embedding a TCanvas in a Qt5 application with ROOT built with Qt4 for example, as reported on the forum: https://root-forum.cern.ch/t/dlopen-error-in-qt-application-when-saving-canvas-as-png/27297

graf2d/gpad/src/TPad.cxx

@@ -4839,7 +4839,7 @@ void TPad::Print(const char *filenam, Option_t *option)
          gPad->GetCanvas()->SetHighLightColor(-1);
          gPad->Modified();
          gPad->Update();
-         if (gVirtualX->InheritsFrom("TGQt")) {
+         if (TClass::GetClass("TGQt", kFALSE) && gVirtualX->InheritsFrom("TGQt")) {
             wid = (this == GetCanvas()) ? GetCanvas()->GetCanvasID() : GetPixmapID();
             gVirtualX->WritePixmap(wid,UtoPixel(1.),VtoPixel(0.),(char *)psname.Data());
          } else {