Use zval_ptr_dtor() to free variables as soon as they hit refcount of 0.

Note:  You should not be using ZVAL_DELREF() in day to day usage.  Instead,
       you should use zval_ptr_dtor().  Use ZVAL_DELREF() only if you're
       messing with the refcount directly and know what you're doing.
Note #2:  For clarity, if you want to initialize a new zval with a refcount
          of 0, it's best to do that directly, instead of using ZVAL_DELREF
          after allocating the zval...

ext/libxml/libxml.c

@@ -466,7 +466,7 @@ PHP_FUNCTION(libxml_set_streams_context)
 		return;
 	}
 	if (LIBXML(stream_context)) {
-		ZVAL_DELREF(LIBXML(stream_context));
+		zval_ptr_dtor(&LIBXML(stream_context));
 		LIBXML(stream_context) = NULL;
 	}
 	ZVAL_ADDREF(arg);