avcodec/dct32_template: Fix runtime error: signed integer overflow: -1071326067 - 1088238847 cannot be represented in type 'int'

Fixes: 1731/clusterfuzz-testcase-minimized-5123972414832640

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