avcodec/fft_template: Fix multiple runtime error: signed integer overflow: -1943918714 - 1935113003 cannot be represented in type 'int'

Fixes: 1735/clusterfuzz-testcase-minimized-5350472347025408

This uses unsigned instead of SUINT because the later was rejected by multiple developers
See: [FFmpeg-devel] [PATCH] avcodec/fft_template: Fix multiple runtime error: signed integer overflow: -1943918714 - 1935113003 cannot be represented in type 'int'

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