avcodec/takdec: Fix integer overflows in decode_subframe()

Fixes: runtime error: signed integer overflow: -1562477869 + -691460395 cannot be represented in type 'int'
Fixes: 3196/clusterfuzz-testcase-minimized-4528307146063872

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