avformat/wtvdec: Check pointer before use

Fixes out of array read
Fixes: 049fdf78565f1ce5665df236d90f8657/asan_heap-oob_10a5a97_1026_42f9d4855547329560f385768de2f3fb.wtv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
