Fixes: signed integer overflow: 9223372036854770375 + 5450 cannot be represented in type 'long'
Fixes: 26471/clusterfuzz-testcase-minimized-ffmpeg_dem_MXG_fuzzer-6229617557635072
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>