mkv.cpp: fix a problem in UnGet()

modules/demux/mkv.cpp

@@ -3335,7 +3335,7 @@ EbmlElement* EbmlParser::UnGet( uint64 i_block_pos, uint64 i_cluster_pos )
     if ( m_el[1]->GetElementPosition() == i_cluster_pos )
     {
         m_es->I_O().setFilePointer( i_block_pos, seek_beginning );
-        return (EbmlMaster*) m_el[mi_level-1];
+        return (EbmlMaster*) m_el[1];
     }
     else
     {