[Coverity 1437930] Potential dereference after null check in matroska_segment_seeker.cpp

Either the check against null is unnecessary, or there may be a null pointer dereference.

In mkv::​virtual_segment_c::​UpdateCurrentToChapter(stream_t &): Pointer is checked against null but then dereferenced anyway (CWE-476)

430    bool b_has_seeked = false;
431
      1. Condition !this->b_current_vchapter_entered, taking true branch.
      2. Condition this->p_current_vchapter != NULL, taking false branch.
      3. var_compare_op: Comparing this->p_current_vchapter to null implies that this->p_current_vchapter might be null.
432    if ( !b_current_vchapter_entered && p_current_vchapter != NULL )
433    {
434        b_current_vchapter_entered = true;
435        if (p_current_vchapter->Enter( true ))
436            return true;
437    }
438
      4. Condition sys.i_pts != 0L, taking true branch.
439    if ( sys.i_pts != VLC_TICK_INVALID )
440    {
      5. Condition this->p_current_vchapter != NULL, taking false branch.
441        if ( p_current_vchapter != NULL && p_current_vchapter->ContainsTimestamp( sys.i_pts - VLC_TICK_0 ))
442            p_cur_vchapter = p_current_vchapter;
      6. Condition p_cur_vedition != NULL, taking true branch.
443        else if (p_cur_vedition != NULL)
444            p_cur_vchapter = p_cur_vedition->getChapterbyTimecode( sys.i_pts - VLC_TICK_0 );
445    }
446
447    /* we have moved to a new chapter */
      7. Condition p_cur_vchapter != NULL, taking true branch.
      8. Condition this->p_current_vchapter != p_cur_vchapter, taking true branch.
448    if ( p_cur_vchapter != NULL && p_current_vchapter != p_cur_vchapter )
449    {
      9. Condition p_cur_vchapter->p_chapter, taking true branch.
450        msg_Dbg( &demux, "New Chapter %" PRId64 " uid=%" PRIu64, sys.i_pts - VLC_TICK_0,
451                 p_cur_vchapter->p_chapter ? p_cur_vchapter->p_chapter->i_uid : 0 );
      10. Condition p_cur_vedition->b_ordered, taking true branch.
452        if ( p_cur_vedition->b_ordered )
453        {
454            /* FIXME EnterAndLeave has probably been broken for a long time */
455            // Leave/Enter up to the link point
     
CID 1437930: (#1 of 1): Dereference after null check (FORWARD_NULL)
11. var_deref_model: Passing null pointer this->p_current_vchapter to EnterAndLeave, which dereferences it.[show details]
456            b_has_seeked = p_cur_vchapter->EnterAndLeave( p_current_vchapter );
457            if ( !b_has_seeked )