Fixes: signed integer overflow: 7 - -2147483647 cannot be represented in type 'int'
Fixes: 28036/clusterfuzz-testcase-minimized-ffmpeg_dem_MICRODVD_fuzzer-5171698751766528

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