avcodec/mss3: Fix runtime error: signed integer overflow: -2146318336 - 2139696256 cannot be represented in type 'int'

Fix is similar to rac_get_model_sym()
Fixes: 1483/clusterfuzz-testcase-minimized-6386507814273024
Fixes: 1485/clusterfuzz-testcase-minimized-6639880215986176

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