awful.client.toggletag: Use tag.getscreen()

Thanks to muni for reporting this bug.

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

lib/awful/client.lua.in

@@ -467,7 +467,7 @@ end
 function client.toggletag(target, c)
     local sel = c or capi.client.focus
     -- Check that tag and client screen are identical
-    if sel and sel.screen == target.screen then
+    if sel and sel.screen == tag.getscreen(target) then
         local tags = sel:tags()
         local index = nil;
         for i, v in ipairs(tags) do