avcodec/takdec: Fix integer overflow

Fixes: runtime error: signed integer overflow: 512 + 2147483146 cannot be represented in type 'int'
Fixes: 2314/clusterfuzz-testcase-minimized-4519333877252096

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