imagebox: Don't try to scale by infinite (FS#1248)

When an imagebox was drawn with width or height zero, it tried to calculate the
needed scale factor for making the image fit. Sadly, this would be a division by
zero aka infinite in this case.

Fix this by just not drawing anything if there is no space available.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/imagebox.lua.in

@@ -18,6 +18,7 @@ local imagebox = { mt = {} }
 --- Draw an imagebox with the given cairo context in the given geometry.
 function imagebox:draw(wibox, cr, width, height)
     if not self._image then return end
+    if width == 0 or height == 0 then return end
 
     cr:save()