Fix handling of UNDEF properties in compound assign

Restore NULLing of UNDEF values in get_property_ptr_ptr for the
BP_VAR_R and BP_VAR_RW cases.

Author: nikic

Zend/tests/type_declarations/typed_properties_103.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Handling of UNDEF property in compound assign 
+--FILE--
+<?php
+class C {
+    public $a = 0;
+}
+function foo() {
+    $x = new C;
+    $x->a = 1;
+    unset($x->a);
+    $x->a += 2;
+    var_dump($x);
+}
+foo();
+?>
+--EXPECTF--
+Notice: Undefined property: C::$a in %s on line %d
+object(C)#1 (1) {
+  ["a"]=>
+  int(2)
+}

Zend/zend_object_handlers.c

@@ -1018,6 +1018,7 @@ ZEND_API zval *zend_std_get_property_ptr_ptr(zval *object, zval *member, int typ
 			if (EXPECTED(!zobj->ce->__get) ||
 			    UNEXPECTED((*zend_get_property_guard(zobj, name)) & IN_GET)) {
 				if (UNEXPECTED(type == BP_VAR_RW || type == BP_VAR_R)) {
+					ZVAL_NULL(retval);
 					zend_error(E_NOTICE, "Undefined property: %s::$%s", ZSTR_VAL(zobj->ce->name), ZSTR_VAL(name));
 				}
 			} else {