Give sorcery instances a reference to their wizards.

On graceful shutdown, sorcery wizards are all killed off, but it is
possible for sorcery instances to still have dangling pointers after
this, possibly causing a crash. Giving the sorcery instances a reference
to their wizards ensures that the wizard reference will remain valid for
the lifetime of the sorcery instance.

Review: https://reviewboard.asterisk.org/r/3401



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@411295 f38db490-d61c-443f-a65b-d21fe96a405b