Fixes: signed integer overflow: 2 * 1210064928 cannot be represented in type 'int'
Fixes: 20873/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5761116909338624

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