Fixes: signed integer overflow: -9223372053736 * 1000000 cannot be represented in type 'long'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_CONCAT_fuzzer-6607924558430208
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 5d7f17e885)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>