awful.client.movetoscreen: Don't untag clients completely (FS#1196)

If a client gets moved to a screen without any selected tags, that client
disappears and it is non-trivial to get it back. Since this is unexpected and
annoying, make movetoscreen do nothing if the target screen has no tags
selected.

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

lib/awful/client.lua.in

@@ -502,8 +502,10 @@ function client.movetoscreen(c, s)
             s = sel.screen + 1
         end
         if s > sc then s = 1 elseif s < 1 then s = sc end
-        sel.screen = s
-        screen.focus(s)
+        if #tag.selectedlist(s) > 0 then
+            sel.screen = s
+            screen.focus(s)
+        end
     end
 end