avcodec/lagarith: fix chroma plane width & height

Fixes out of array read
Fixes: asan_heap-oob_1bf48fa_2513_lag-yuy2.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
