sw: fix crash in InitBookmarks() with forum-mso-en-8410.docx

Error: attempt to dereference a past-the-end iterator.

Probably this only crashes in the assert(iter->pNode...) as the rest of
the function already checks that "iter == end".

(regression from commit 46e04a712e)

Change-Id: If652a15d1cdebb21fbce028fceeaee8dce8f7f6d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/138205
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/text/porlay.cxx

@@ -1051,6 +1051,10 @@ static void InitBookmarks(
                 break;
             }
         }
+        if (iter == end)
+        {
+            break; // remaining marks are hidden
+        }
     }
 }