Fix: Exit early from update_orthos() when orthos are off

[mrariden]

When the orthos were off in the 3d gui and update_orthos() was called, an error would occur because zmax was unset:

cellpose/cellpose/gui/gui3d.py

Lines 526 to 529 in c69c4ae

	zrange = zmax - zmin
	self.layer_ortho = [
	np.zeros((self.Ly, zrange, 4), "uint8"),
	np.zeros((zrange, self.Lx, 4), "uint8")

The fix is to return from update_orthos() early when the orthobtn is unchecked. I just added a guard clause and unindented the block in the first if statement.

• Gui testing protocol
• Tests passing