avcodec/vmdvideo: Check len before using it in method 3

      Fixes out of array access
Fixes: asan_heap-oob_4d23ba_91_cov_3853393937_128.vmd

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