demux: mkv: fix single track opus playback
demux: mkv: fix single track opus playback

Only first frame in a block has timestamp, then (0) last_dts is never valid,
(1) PCR never sent, (2) track never selected by core, goto 1;

xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc