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

@@ -502,9 +502,11 @@ function client.movetoscreen(c, s)
             s = sel.screen + 1
         end
         if s > sc then s = 1 elseif s < 1 then s = sc end
+        if #tag.selectedlist(s) > 0 then
             sel.screen = s
             screen.focus(s)
         end
+    end
 end
 
 --- Mark a client, and then call 'marked' hook.