mkv.cpp: better test when files with no Segment UID are in the same dir

modules/demux/mkv.cpp

@@ -5223,7 +5223,9 @@ size_t virtual_segment_c::AddSegment( matroska_segment_c *p_segment )
     // check if it's not already in here
     for ( i=0; i<linked_segments.size(); i++ )
     {
-        if ( linked_segments[i]->p_segment_uid != NULL && *p_segment->p_segment_uid == *linked_segments[i]->p_segment_uid )
+        if ( linked_segments[i]->p_segment_uid != NULL 
+			&& p_segment->p_segment_uid != NULL
+			&& *p_segment->p_segment_uid == *linked_segments[i]->p_segment_uid )
             return 0;
     }