Related: tdf#105707 PPTX import: warn on invalid SmartArt fallback relid

We already warn when the SmartArt doesn't have a drawingML fallback, but
having a reference to a fallback that is not a valid one (so resolves to
an empty fragment path) is also unusable. So warn in that case as well.

Change-Id: I03b2325003b4eb38b1347a88899799c6d5c33606
Reviewed-on: https://gerrit.libreoffice.org/34129Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

oox/source/drawingml/graphicshapecontext.cxx

@@ -275,6 +275,18 @@ ContextHandlerRef DiagramGraphicDataContext::onCreateContext( ::sal_Int32 aEleme
         // No DrawingML fallback, need to warn the user at the end.
         if (mpShapePtr->getExtDrawings().empty())
             getFilter().setMissingExtDrawing();
+        else
+        {
+            for (const auto& rRelId : mpShapePtr->getExtDrawings())
+            {
+                // An invalid fallback reference is as bad as a missing one.
+                if (getFragmentPathFromRelId(rRelId).isEmpty())
+                {
+                    getFilter().setMissingExtDrawing();
+                    break;
+                }
+            }
+        }
 
         break;
     }