avformat/webvttdec: Dont skip over a never checked byte after -->

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f49667d83db_3396_WebVTT_capability_tester.vtt
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>