Fix seldom crash in TPad::ShowGuidelines

In some cases gPad can be set to tmpGuideLinePad, which is deleted
any time ShowGuidelines method is called. In that case pointer on
this temporary pad should be removed from TContext object

graf2d/gpad/src/TPad.cxx

@@ -6240,15 +6240,16 @@ void TPad::ShowGuidelines(TObject *object, const Int_t event, const char mode, c
    if (mode != 'i') resize = true;
 
    TPad *is_pad = dynamic_cast<TPad *>( object );
-   TVirtualPad *padSave = 0;
-   padSave = gPad;
+   TVirtualPad *padSave = gPad;
    if (is_pad)
      if (is_pad->GetMother()) is_pad->GetMother()->cd();
 
-   static TPad * tmpGuideLinePad;
+   static TPad *tmpGuideLinePad = 0;
 
    //delete all existing Guidelines and create new invisible pad
    if (tmpGuideLinePad) {
+      if (padSave == tmpGuideLinePad)
+         padSave = 0;
       if (object == tmpGuideLinePad) { // in case of funny button click combination.
          tmpGuideLinePad->Delete();
          tmpGuideLinePad = 0;
@@ -6502,7 +6503,7 @@ void TPad::ShowGuidelines(TObject *object, const Int_t event, const char mode, c
    }
 
    gPad->Modified(kTRUE);
-   padSave->cd();
+   if (padSave) padSave->cd();
 }
 
 ////////////////////////////////////////////////////////////////////////////////