avcodec/apedec: Check length in long_filter_high_3800()

Fixes out of array read
Fixes: 0a7ff0c1d93da9cef28a315ec91b692a/asan_heap-oob_4a52e5_3604_9c56dbb20e308f4faeef7b35f688521a.ape

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