use NULL instead of 0

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52369 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -1142,7 +1142,7 @@ rb_vm_cref(void)
     rb_thread_t *th = GET_THREAD();
     rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(th, th->cfp);
 
-    if (cfp == 0) {
+    if (cfp == NULL) {
 	return NULL;
     }