avcodec/takdec: Fix integer overflow in decode_subframe()

Fixes: runtime error: signed integer overflow: -536870912 - 1972191120 cannot be represented in type 'int'
Fixes: 2711/clusterfuzz-testcase-minimized-4975142398590976

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
