avcodec/takdec: Fix multiple runtime error: signed integer overflow: -512 * 4563386 cannot be represented in type 'int'

Fixes: 1706/clusterfuzz-testcase-minimized-6112772670619648

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