avformat/avidec: Use a buffer with sufficient padding in read_gab2_sub()

Fixes out of array read
Fixes: 0ff9841c2a102f06e0d582bfc3376cbd-asan_heap-oob_495589_6836_cov_1763916974_mewmew_ssa.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
