Geom: nil guard a few rect methods (#7664)

We've managed to trip a few of those on dimen fields post-init but
pre-paintTo in a few weird coner-cases, a point at which dimen is often
nil.

ConfigDialog: Deal with that very thing in update()

Fix #7656

spec/unit/device_spec.lua

@@ -9,7 +9,7 @@ describe("device module", function()
         mock_fb = {
             new = function()
                 return {
-                    getSize = function() return {w = 600, h = 800} end,
+                    getRawSize = function() return {w = 600, h = 800} end,
                     getWidth = function() return 600 end,
                     getDPI = function() return 72 end,
                     setViewport = function() end,