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.