Fix possible missing parameter in playlist url extraction

The "playlist_prefix" parameter was missing when parsing playlist urls
that match the recently added format, e.g.:
http://www.youtube.com/user/stanforduniversity#g/c/9D558D49CA734A02
For these URLs (basically, for every playlist type so far, except the
artist list) playlist_prefix has to be equal to "p" for correct
exctraction.

youtube-dl

@@ -2135,6 +2135,7 @@ class YoutubePlaylistIE(InfoExtractor):
 		if playlist_prefix == 'a':
 			playlist_access = 'artist'
 		else:
+			playlist_prefix = 'p'
 			playlist_access = 'view_play_list'
 		playlist_id = mobj.group(2)
 		video_ids = []