ALSA uses a global config structure that's overwritten (and not
free'd) every time one of those functions is called, so we have
to manually call snd_config_update_free_global() to release it.

Hint taken from MEMORY-LEAK in the ALSA source code

git-svn-id: https://svn.musicpd.org/mpd/trunk@4381 09075e82-0dd4-0310-85a5-a0d7c8717e4f