avcodec/takdec: Fix runtime error: signed integer overflow: 2146548196 + 2156738 cannot be represented in type 'int'

Fixes: 1743/clusterfuzz-testcase-minimized-4994834022531072

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