avoid undefined null-reference in CrossRefBookmark::GetOtherMarkPos()

CrossRefBookmark::GetOtherMarkPos() is apparently not supposed to be
called, so just replace the undefined null-reference with a call to
abort().

Change-Id: Ie8e141d661b870573092d20bd85fa1ae04506421

sw/source/core/crsr/crossrefbookmark.cxx

@@ -59,7 +59,7 @@ namespace sw { namespace mark
         OSL_PRECOND(false,
             "<sw::mark::CrossRefBookmark::GetOtherMarkPos(..)>"
             " - this should never be called!");
-        return *static_cast<SwPosition*>(NULL);
+        abort();
     }
 
     CrossRefHeadingBookmark::CrossRefHeadingBookmark(const SwPaM& rPaM,