Avoid crash in security driver if model is NULL

If the XML security model is NULL, it is assumed that the current
model will be used with dynamic labelling. The verify step is
meaningless and potentially crashes if dereferencing NULL

* src/security/security_manager.c: Skip NULL model on verify

src/security/security_manager.c

@@ -309,6 +309,14 @@ int virSecurityManagerSetProcessLabel(virSecurityManagerPtr mgr,
 int virSecurityManagerVerify(virSecurityManagerPtr mgr,
                              virDomainDefPtr def)
 {
+    const virSecurityLabelDefPtr secdef = &def->seclabel;
+    /* NULL model == dynamic labelling, with whatever driver
+     * is active, so we can short circuit verify check to
+     * avoid drivers de-referencing NULLs by accident
+     */
+    if (!secdef->model)
+        return 0;
+
     if (mgr->drv->domainSecurityVerify)
         return mgr->drv->domainSecurityVerify(mgr, def);