avformat/iff: simplify duration calculation

Fixes: signed integer overflow: 315680096256 * 134215943 cannot be represented in type 'long long'
Fixes: 48713/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-5886272312311808

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 0740641e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>