avcodec/ccaption_dec: Use simple array instead of AVBuffer

This is simpler and fixes an out of array read, fixing it with AVBuffers
would be more complex

Fixes: e00d9e6e50e5495cc93fea41147b97bb/asan_heap-oob_12dcdbb_8798_b32a97ea722dd37bb5066812cc674552.mov

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