The function spl_valid_name() should verify playlist names and prevent
path traversal, but it failed to do so on Windows, because it forgot
to check for backslashes.

This buggy piece of code was already present when stored playlists
were initially implemented in 2006 by commit 08003904, and
even during the many rounds of code refactoring, nobody ever bothered
to verify it.  D'oh!

(Thanks, Paul Arzelier)