avcodec/mdct_*: Fix integer overflow in addition in RESCALE()

Fixes: runtime error: signed integer overflow: 1219998458 - -1469874012 cannot be represented in type 'int'
Fixes: 3443/clusterfuzz-testcase-minimized-5369987105554432

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