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: https://origsvn.digium.com/svn/asterisk/branches/13@424448 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/sorcery.c

@@ -1572,6 +1572,12 @@ static int sorcery_wizard_create(void *obj, void *arg, int flags)
 	const struct ast_sorcery_object_wizard *object_wizard = obj;
 	const struct sorcery_details *details = arg;
 
+	if (!object_wizard->wizard->create) {
+		ast_assert(0);
+		ast_log(LOG_ERROR, "Sorcery wizard '%s' doesn't contain a 'create' virtual function.\n",
+			object_wizard->wizard->name);
+		return 0;
+	}
 	return (!object_wizard->caching && !object_wizard->wizard->create(details->sorcery, object_wizard->data, details->obj)) ? CMP_MATCH | CMP_STOP : 0;
 }