avformat/sdsdec: Use av_rescale() to avoid intermediate overflow in duration calculation

Fixes: signed integer overflow: 72128794995445727 * 240 cannot be represented in type 'long'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_SDS_fuzzer-6628185583779840

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