Fixes: signed integer overflow: -4611686024827895807 + -4611686016279904256 cannot be represented in type 'long'
Fixes: 30161/clusterfuzz-testcase-minimized-ffmpeg_dem_R3D_fuzzer-5694406713802752

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>