Attempt to blind fix tdf#108612 explicitly checking for clipboard document

It seems that for some yet unknown reason in an (un)certain constellation
ScDocument::IsClipboardSource() returns true because ScModule::GetClipDoc()
does not return the clipboard document but the actual document instead, so
destroying the clipboard document executes ScDocument::ClosingClipboardSource()
and (or in earlier versions) ScDocument::ForgetNoteCaptions() on the actual
document instead.

Check also that ScDocument::bIsClip is set, hopefully curing the symptom, the
real cause is something else.

Change-Id: I5ffd0706592d2737808e7004630a56c53115295b

sc/source/core/data/document.cxx

@@ -2543,7 +2543,7 @@ void ScDocument::SetClipParam(const ScClipParam& rParam)
 bool ScDocument::IsClipboardSource() const
 {
     ScDocument* pClipDoc = ScModule::GetClipDoc();
-    return xPoolHelper.is() && pClipDoc && pClipDoc->xPoolHelper.is() &&
+    return pClipDoc && pClipDoc->bIsClip && pClipDoc->xPoolHelper.is() && xPoolHelper.is() &&
             xPoolHelper->GetDocPool() == pClipDoc->xPoolHelper->GetDocPool();
 }