demux: mp4: fix double duration with fragmented
demux: mp4: fix double duration with fragmented

Happens with duration on tkhd but empty moov

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc