ucm: initialize mgr->once_list

alsa-project/alsa-lib pull request #50 was opened from asavah:

Initiliaze mgr->once_list otherwise eg `alsactl restore 0` will segfault in https://github.com/alsa-project/alsa-lib/blob/master/src/ucm/utils.c#L570 in case that `/var/lib/alsa/alsa.state` is not present.

Request URL : https://github.com/alsa-project/alsa-lib/pull/50 Patch URL : https://github.com/alsa-project/alsa-lib/pull/50.patch Repository URL: https://github.com/alsa-project/alsa-lib