Avoid crash when calling a Worksheet's ShowDataForm()

in VBA, e.g.:
Option VBASupport 1
Sub Main
    Worksheets(1).ShowDataForm()
End Sub

Change-Id: Ice6cfccc4fc15a4d0044d6464c7d6c04243e86ae
Reviewed-on: https://gerrit.libreoffice.org/30619Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/ui/vba/vbaworksheet.cxx

@@ -900,8 +900,8 @@ ScVbaWorksheet::ShowDataForm( ) throw (uno::RuntimeException, std::exception)
     ScAbstractDialogFactory* pFact = ScAbstractDialogFactory::Create();
     OSL_ENSURE(pFact, "ScAbstractFactory create fail!");
 
-    AbstractScDataFormDlg* pDlg = pFact->CreateScDataFormDlg(pTabViewShell->GetDialogParent(),
-        pTabViewShell);
+    ScopedVclPtr<AbstractScDataFormDlg> pDlg(pFact->CreateScDataFormDlg(pTabViewShell->GetDialogParent(),
+                                                                        pTabViewShell));
     OSL_ENSURE(pDlg, "Dialog create fail!");
 
     pDlg->Execute();