mpris-widget: correctly clean list of players

Do it outside the loop, otherwise the last player wins.

mpris-widget/init.lua

@@ -72,9 +72,9 @@ local popup = awful.popup{
 
 local function rebuild_popup()
     awful.spawn.easy_async(LIST_PLAYERS_CMD, function(stdout, _, _, _)
+        for i = 0, #rows do rows[i]=nil end
         for player_name in stdout:gmatch("[^\r\n]+") do
             if player_name ~='' and player_name ~=nil then
-                for i = 0, #rows do rows[i]=nil end
 
                 local checkbox = wibox.widget{
                     {