Extend check_obj.cocci to the trivial base case

does not apply in varnish-cache, but found useful in a vmod

tools/coccinelle/check_obj.cocci

 /*
- * This patch removes a redundant null check.
+ * This patch removes a redundant null check and replaces assertions
+ * on the magic with CHECK_OBJ()
  */
 
 @@
@@ -11,3 +12,10 @@ if (obj != NULL) {
 + CHECK_OBJ(obj, magic);
 ...
 }
+
+@@
+expression obj, magicval;
+@@
+
+- assert(obj->magic == magicval);
++ CHECK_OBJ(obj, magicval);