awful.screen: Fix addition between object and number

Fix #784

lib/awful/client.lua

@@ -559,7 +559,7 @@ function client.movetoscreen(c, s)
     if sel then
         local sc = capi.screen.count()
         if not s then
-            s = sel.screen + 1
+            s = sel.screen.index + 1
         end
         if type(s) == "number" and s > sc then s = 1 elseif s < 1 then s = sc end
         s = get_screen(s)

lib/awful/screen.lua

@@ -143,7 +143,7 @@ end
 -- @param i Value to add to the current focused screen index. 1 will focus next
 -- screen, -1 would focus the previous one.
 function screen.focus_relative(i)
-    return screen.focus(util.cycle(capi.screen.count(), screen.focused() + i))
+    return screen.focus(util.cycle(capi.screen.count(), screen.focused().index + i))
 end
 
 --- Get or set the screen padding.