Simplify band splitting

[radarhere]

When splitting an image into multiple bands,

Pillow/src/_imaging.c

Lines 2473 to 2484 in b33a315

	for (i = 0; i < self->image->bands; i++) {
	imaging_object = PyImagingNew(bands[i]);
	if (!imaging_object) {
	fails += 1;
	}
	PyTuple_SET_ITEM(list, i, imaging_object);
	}
	if (fails) {
	Py_DECREF(list);
	list = NULL;
	}
	return list;

rather than counting failures and continuing, it would simpler to break immediately on failure.