demux: mp4: handle broken ismv trun data offset
demux: mp4: handle broken ismv trun data offset

Some smooth streaming implicit trun data offset are
pointing to (seems) tfhd 1st byte, instead of moof,
resulting of first samples pointing inside moof.

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