awful.screen.object.get_dpi: Ignore outputs with size 0 (#2063)

For example, Xephyr reports its output with a size of 0x0. Since a
division by zero is in no one's interest, just ignore such outputs when
trying to compute the DPI value.

Thanks to @timroes for pointing this out:
c8fac753c4 (commitcomment-25072296)

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

lib/awful/screen.lua

@@ -529,10 +529,13 @@ function screen.object.get_dpi(s)
     local dpi = nil
     local geo = s.geometry
     for _, o in pairs(s.outputs) do
+        -- Ignore outputs with width/height 0
+        if o.mm_width ~= 0 and o.mm_height ~= 0 then
             local dpix = geo.width * mm_per_inch / o.mm_width
             local dpiy = geo.height * mm_per_inch / o.mm_height
             dpi = math.min(dpix, dpiy, dpi or dpix)
         end
+    end
     if dpi then
         return dpi
     end