demux: mkv: fix seek

This fixes a regression from 6b10c2e6. Seek failed due to empty
_tracks_seekpoints[video_track].

modules/demux/mkv/matroska_segment.cpp

@@ -635,6 +635,16 @@ bool matroska_segment_c::Preload( )
 
 
             cluster = kc_ptr;
+
+            // add first cluster as trusted seekpoint for all tracks
+            for( tracks_map_t::const_iterator it = tracks.begin();
+                 it != tracks.end(); ++it )
+            {
+                _seeker.add_seekpoint( it->first,
+                SegmentSeeker::Seekpoint( cluster->GetElementPosition(), -1,
+                                          SegmentSeeker::Seekpoint::TrustLevel::QUESTIONABLE ) );
+            }
+
             /* stop pre-parsing the stream */
             break;
         }