avcodec/xface: correct the XFACE_MAX_* values

      Fixes out of array access

Fixes: asan_stack-oob_32c12e5_2536_cov_2442316831_lena.xface
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>