avcodec/h264: Check mode before considering mixed mode intra prediction

Fixes out of array read
Fixes: asan_heap-oob_e476fc_2_asan_heap-oob_1333ec6_61_CAMACI3_Sony_C.jsv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>