php扩展中FREE_ZVAL和zval_ptr_dtor的使用条件？

zval *z;
MAKE_STD_ZVAL(z);
ZVAL_STRING(z, "this is a test", 0);

zval_ptr_dtor(&z); 
//这里会报错
 Program received signal SIGSEGV, Segmentation fault.
0x000000000067a01d in zval_delref_p (pz=0x7ffff063f323) at /vagrant_data/php-5.6.25/Zend/zend.h:411
411        return --pz->refcount__gc;