[gui] avoid loading of TStyleManager for quit ROOT

When selecting "Quit ROOT" menu item in canvas, do not load TStyleManager class.
Only if it was loaded before - execute Terminate() method

Author: linev

gui/gui/src/TRootCanvas.cxx

@@ -950,7 +950,7 @@ Bool_t TRootCanvas::ProcessMessage(Longptr_t msg, Longptr_t parm1, Longptr_t)
                      }
                      if (TVirtualPadEditor::GetPadEditor(kFALSE) != 0)
                         TVirtualPadEditor::Terminate();
-                     if (TClass::GetClass("TStyleManager"))
+                     if (TClass::GetClass("TStyleManager", kFALSE, kTRUE))
                         gROOT->ProcessLine("TStyleManager::Terminate()");
                      gApplication->Terminate(0);
                      break;