Ensure that furthest v is set in quantize2

[radarhere]

First, in quantize2, the variable Pixel furthest is only used for its v property. So the Pixel can be simplified to just that uint32_t.

Second, quantize2 sets furthestDistance to zero,

Pillow/src/libImaging/Quant.c

Lines 1579 to 1583 in 31800a0

	data.furthestDistance = 0;
	data.secondPixel = (i == 1) ? 1 : 0;
	hashtable_foreach_update(h, compute_distances, &data);
	p[i].v = data.furthest.v;
	data.new.v = data.furthest.v;

and then runs compute_distances, assigning furthest,

Pillow/src/libImaging/Quant.c

Lines 1528 to 1541 in 31800a0

	compute_distances(const HashTable *h, const Pixel pixel, uint32_t *dist, void *u) {
	DistanceData *data = (DistanceData *)u;
	uint32_t oldDist = *dist;
	uint32_t newDist;
	newDist = _DISTSQR(&(data->new), &pixel);
	if (data->secondPixel || newDist < oldDist) {
	*dist = newDist;
	oldDist = newDist;
	}
	if (oldDist > data->furthestDistance) {
	data->furthestDistance = oldDist;
	data->furthest.v = pixel.v;
	}
	}

before it returns to quantize2 and p[i].v and data.new.v are set to data.furthest.v.

What if oldDist was never greater than zero though? Then furthest.v would never be assigned.
This causes a problem in python-pillow/docker-images#148 - /~https://github.com/python-pillow/docker-images/runs/6247349541?check_suite_focus=true.
So instead, this PR sets it to the first v of pixelData.