fix: crash on some clang

[aged-urchin]

when compiled by some clang(the compiler used by webrtc m148), program crash on expressions of "FFTSkew - 1".
(the compiler doesn't actually evaluate the expression, but simpily crash because of undefined behavior of array index out of bounds）, changing it to" (ffe_t*)FFTSkew - 1" could effectively avoid this crash.

[BenBE]

Interestingly, accessing any element of FFTSkew outside of its boundaries is UB, And while reasoning about this might show the intend for all acceses done through these pointers to be in-bounds, I don't think we necessarily have this guarntee here. At least raises an eyebrow for me, while looking at the code.

Thus while FFTSkew - 1 is a valid expression (FFTSkew may decay to ffe_t*, thus yielding pointer arithmetic), it makes reasoning about the code unnecessarily complicated. Is this even Valgrind-clean?

The proposed patch is a no-op regarding the underlying UB, even if it "fixes" the immediate crash of clang.