demux:mkv: don't check if a track is selected if it has no ES

modules/demux/mkv/matroska_segment.cpp

@@ -801,7 +801,10 @@ bool matroska_segment_c::Seek( demux_t &demuxer, vlc_tick_t i_absolute_mk_date,
         track.i_last_dts        = VLC_TS_INVALID;
 
         bool selected;
-        es_out_Control( demuxer.out, ES_OUT_GET_ES_STATE, track.p_es, &selected );
+        if (track.p_es == NULL)
+            selected = false;
+        else
+            es_out_Control( demuxer.out, ES_OUT_GET_ES_STATE, track.p_es, &selected );
         if ( selected )
             selected_tracks.push_back( track.i_number );
     }