demux:mkv: always start from the first Cluster when we don't know we to seek

When the file is truncated some previous seek may have failed and we need to
reset the OEF.

modules/demux/mkv/matroska_segment_seeker.cpp

@@ -486,6 +486,11 @@ SegmentSeeker::mkv_jump_to( matroska_segment_c& ms, fptr_t fpos )
             mark_range_as_searched( Range( i_cluster_pos, ms.es.I_O().getFilePointer() ) );
         }
     }
+    else if (ms.cluster != NULL)
+    {
+        // make sure we start reading after the Cluster start
+        ms.es.I_O().setFilePointer(ms.cluster->GetDataStart());
+    }
 
     ms.ep.Down();