avcodec/dxtory: Fix input size check in dxtory_decode_v1_420()

Fixes out of array read
Fixes: c50c4aa6cefda71b19a31ea12302980c/asan_heap-oob_12be5fd_7011_33ebd015a74976215934add72b9c8352.avi

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