avcodec/apedec: Fix 2 integer overflows in filter_3800()

Fixes: signed integer overflow: 1683879955 - -466265224 cannot be represented in type 'int'
Fixes: 37419/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APE_fuzzer-6074294407921664

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>