ttml demux: Fix read of uninitialized pointer
ttml demux: Fix read of uninitialized pointer

Introduced in 4ed089f8
Fix cid #1372955

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