res_pjsip_config_wizard: fix unload SEGV

If certain pjsip modules aren't loaded, the wizard causes a SEGV
when it unloads.  Added a check for the presense of the object
type wizard before trying to clean it up.

Tested-by: George Joseph



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@429719 f38db490-d61c-443f-a65b-d21fe96a405b