coverity#982488 unchecked dynamic_cast

Change-Id: I7008b3a96400a4d1520a03d985c0b421ff587bcb

unoxml/source/dom/document.cxx

@@ -441,6 +441,7 @@ namespace DOM
         ::rtl::Reference< CAttr > const pCAttr(
             dynamic_cast< CAttr* >(GetCNode(
                     reinterpret_cast<xmlNodePtr>(pAttr)).get()));
+        if (!pCAttr.is()) { throw RuntimeException(); }
         pCAttr->m_bUnlinked = true;
         return pCAttr.get();
     };