avcodec/sbrdsp_fixed: Fix integer overflow

Fixes: signed integer overflow: 2147483598 + 64 cannot be represented in type 'int'
Fixes: 4337/clusterfuzz-testcase-minimized-6192658616680448

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