sorcery: Prevent SEGV in sorcery_wizard_create when there's no create function

When you call ast_sorcery_create() you don't necessarily know which wizard is
going to be invoked.  If it happens to be a wizard like 'config' that doesn't
have a 'create' virtual function you get a segfault in the
sorcery_wizard_create callback.  This patch catches the null function pointer,
does an ast_assert, and logs an error.

Review: https://reviewboard.asterisk.org/r/4044/
........

Merged revisions 424447 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424448 f38db490-d61c-443f-a65b-d21fe96a405b