sw: fix SwAccessibleParagraph crash on delete

It happens that SwAccessibleParagraph::getCharacterBounds() is called
from ATK with a clearly invalid pPortionData member; the SwTxtNode has
no text but there are portions.

This is because after a deletion both a POS_CHANGED and a
INVALID_CONTENT event are created for the same SwAccessibleParagraph,
and they are merged into one POS_CHANGED event by
SwAccessibleMap::AppendEvent(), but
SwAccessibleContext::InvalidatePosOrSize() returns early if the
paragraph happens to not be visible, skipping the invalidation.

(regression from 76c549eb, which
 removed a Dispose() call; not sure if the Dispose() is needed here)

Change-Id: I985e9a439ee6c7024963eace876186f2247b9e03

sw/source/core/access/acccontext.cxx

@@ -1160,12 +1160,9 @@ void SwAccessibleContext::InvalidatePosOrSize( const SwRect& )
         FireVisibleDataEvent();
     }
 
-    if( !bIsNewShowingState &&
-        SwAccessibleChild( GetParent() ).IsVisibleChildrenOnly() )
-    {
-        return;
-    }
-
+    // note: InvalidatePosOrSize must call _InvalidateContent so that
+    // SwAccessibleParagraph updates its portions, or dispose it
+    // (see accmap.cxx: INVALID_CONTENT is contained in POS_CHANGED)
     _InvalidateContent( true );
 }