Fixes: signed integer overflow: 2147483647 + 64 cannot be represented in type 'int'
Fixes: 30333/clusterfuzz-testcase-minimized-ffmpeg_dem_RM_fuzzer-5175286983426048

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