assert magic of vrt_privs when finalizing

Originally suggested by @rezan in
https://github.com/varnishcache/varnish-cache/pull/3408/commits/4caad2c1858afb399f75230916183d31f57fc908

bin/varnishd/cache/cache_vrt_priv.c

@@ -238,7 +238,9 @@ VCL_TaskLeave(struct vrt_privs *privs)
 	 * a costly operation. Instead we safely walk the whole tree and clear
 	 * the head at the very end.
 	 */
-	VRBT_FOREACH_SAFE(vp, vrt_privs, privs, vp1)
+	VRBT_FOREACH_SAFE(vp, vrt_privs, privs, vp1) {
+		CHECK_OBJ(vp, VRT_PRIV_MAGIC);
 		VRT_priv_fini(vp->priv);
+	}
 	ZERO_OBJ(privs, sizeof *privs);
 }