Add check for positive xsize/ysize in ImagingFliDecode

[Yay295]

Fixes #8405

I put the check near the top, but is there any valid reason xsize or ysize would be 0, and the check would need to be later in the code?

[radarhere]

is there any valid reason xsize or ysize would be 0

No. Images that say one of their dimensions are zero will be stopped at

Pillow/src/PIL/ImageFile.py

Lines 154 to 156 in 731bcda

	if not self.mode or self.size[0] <= 0 or self.size[1] <= 0:
	msg = "not identified by this driver"
	raise SyntaxError(msg)

Even if you consider just the C decoding process, we have

Pillow/src/decode.c

Lines 189 to 192 in 731bcda

	if (state->xsize <= 0 || state->xsize + state->xoff > (int)im->xsize ||
	state->ysize <= 0 || state->ysize + state->yoff > (int)im->ysize) {
	PyErr_SetString(PyExc_ValueError, "tile cannot extend outside image");
	return NULL;

[radarhere]

My minor concern with this strategy is that we could make someone reading the code think that xsize might be zero, which as I've said, shouldn't happen.

I've created #8408 as an alternative to this.