codec/substtml: fix inifinite loop on unexpected data
codec/substtml: fix inifinite loop on unexpected data

fixes #17403

Signed-off-by: Hugo Beauzée-Luyssen <3faf7ed52fa83d583fc670a96bcf92da270d0767@beauzee.fr>

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