Only check real files.

diff --git a/pl.py b/pl.py
index be80846..fa19b75 100755
--- a/pl.py
+++ b/pl.py
@@ -150,7 +150,7 @@ def main():
     if action == 'check':
         bad_playlists = []
         for playlist in playlists:
-            if not playlist_is_ok(playlist, options):
+            if os.path.isfile(playlist) and not playlist_is_ok(playlist, options):
                 bad_playlists.append(playlist)
                 if options.error_stop:
                     break