Return 0 or -1 from ImagingSplit(), rather than number of bands

[radarhere]

The only time that ImagingSplit() is called, the result is only checked for success or failure.

Pillow/src/_imaging.c

Lines 2507 to 2509 in 0253ef0

	if (!ImagingSplit(self->image, bands)) {
	return NULL;
	}

There's no reason it needs to return the number of bands.

Pillow/src/libImaging/Bands.c

Line 173 in 0253ef0

return imIn->bands;

So let's return 0 for success and -1 for failure instead.