imagebox: Refuse invalid images

This makes the imagebox widget refuse images which have a zero or negative width
or height. A zero size causes problems where a division by zero results in inf
which makes various stuff fail in weird ways later on.

Additionally, cairo's "error surfaces" have a negative size. Displaying those
would cause all sorts of weird problems, too, so we better reject those.

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

lib/wibox/widget/imagebox.lua.in

@@ -82,6 +82,14 @@ function set_image(box, image)
         image = result
     end
 
+    if image then
+        local w = image:get_width()
+        local h = image:get_height()
+        if w <= 0 or h <= 0 then
+            return false
+        end
+    end
+
     box.image = image
 
     box:emit_signal("widget::updated")