awful.tag.setscreen: Check if old_screen == new_screen (FS#1249)

Setting a tag's screen to what it already is shouldn't have any bad effects.
However, this code messed up the tag order and selection status.

Fix this by returning early if the tag already has the right screen.

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

lib/awful/tag.lua.in

@@ -271,12 +271,12 @@ end
 -- @param s Screen number
 function tag.setscreen(t, s)
     local s = s or capi.mouse.screen
+    local old_screen = tag.getproperty(t, "screen")
+    if s == old_screen then return end
 
     -- Keeping the old index make very little sense when changing screen
     tag.setproperty(t, "index", nil)
 
-    local old_screen = tag.getproperty(t,"screen")
-
     -- Change the screen
     tag.setproperty(t, "screen", s)