Fix compiler warning: accessing 64 bytes in a region of size 48

[wiredfool]

  src/_imaging.c:1842:17: warning: ‘ImagingTransform’ accessing 64 bytes in a region of size 48 [-Wstringop-overflow=]
   1842 |         imOut = ImagingTransform(
        |                 ^~~~~~~~~~~~~~~~~
   1843 |             imOut, imIn, IMAGING_TRANSFORM_AFFINE, 0, 0, xsize, ysize, a, filter, 1);
        |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/_imaging.c:1842:17: note: referencing argument 8 of type ‘double *’

[Yay295]

Just fyi, this isn't actually an error per se. ImagingTransform takes an "a" that is double[8], but ImagingTransformAffine takes a double[6], so passing a double[6] here is okay. I'd written a comment about this in my local code but hadn't committed it yet.

[wiredfool]

Yeah, I'd run through the analysis of if it was an issue or not and came to the conclusion that it was not a memory safety issue, so not a security problem. It's still a warning that should be silenced, at a cost of 2 extra doubles on the stack.