Fix another ref source management bug in unserialize

When we overwrite an existing property during unserialization,
we also have to drop the ref source from it.

Author: nikic

ext/standard/tests/serialize/typed_property_ref_overwrite.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Overwriting a typed property reference
+--FILE--
+<?php
+
+class Test {
+    public ?object $prop;
+}
+$s = <<<'STR'
+O:4:"Test":2:{s:4:"prop";R:1;s:4:"prop";N;}}
+STR;
+var_dump(unserialize($s));
+
+?>
+--EXPECT--
+object(Test)#1 (1) {
+  ["prop"]=>
+  NULL
+}

ext/standard/var_unserializer.re

@@ -556,6 +556,10 @@ string_key:
 						/* This is a property with a declaration */
 						old_data = Z_INDIRECT_P(old_data);
 						info = zend_get_typed_property_info_for_slot(obj, old_data);
+						if (Z_ISREF_P(old_data)) {
+							/* If the value is overwritten, remove old type source from ref. */
+							ZEND_REF_DEL_TYPE_SOURCE(Z_REF_P(old_data), info);
+						}
 						var_push_dtor(var_hash, old_data);
 						Z_TRY_DELREF_P(old_data);
 						ZVAL_COPY_VALUE(old_data, &d);