Use correct bands for two band histograms

[radarhere]

Resolves #9050

For an image32 image without a mask, histogram values are generated with

Pillow/src/libImaging/Histo.c

Lines 136 to 139 in 37cd041

	h->histogram[(*in++)]++;
	h->histogram[(*in++) + 256]++;
	h->histogram[(*in++) + 512]++;
	h->histogram[(*in++) + 768]++;

However, this doesn't consider that some images (PA, LA and La) only have two bands

Pillow/src/libImaging/Storage.c

Lines 78 to 80 in 37cd041

	} else if (strcmp(mode, "PA") == 0) {
	/* 8-bit palette with alpha */
	im->bands = 2;

Pillow/src/_imaging.c

Lines 1360 to 1361 in 37cd041

	/* Build an integer list containing the histogram */
	list = PyList_New(h->bands * 256);

meaning that two band histograms only have 512 values, rather than 1024. So this PR skips the middle two bands when assigning values.