serialize: Fixed handling of nested object references

The removed if in var.c caused serialize to not handle object references
correctly under certain circumstances.

See tests/serialize/serialization_objects_019.phpt

The bug was originally introduced in commit 6c5942f, and the problematic
line was last modified in commit bb0b4eb. (Fixes oss-fuzz #44954)

The testcase from bb0b4eb still passes.

Author: Martin Hoch

ext/standard/tests/serialize/serialization_objects_019.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Object serialization with references
+--FILE--
+<?php
+function gen() {
+    $s = new stdClass;
+    $r = new stdClass;
+    $r->a = [$s];
+    $r->b = $r->a;
+    return $r;
+}
+var_dump(serialize(gen()));
+?>
+--EXPECTF--
+string(78) "O:8:"stdClass":2:{s:1:"a";a:1:{i:0;O:8:"stdClass":0:{}}s:1:"b";a:1:{i:0;r:3;}}"
+
+

ext/standard/var.c

@@ -666,8 +666,6 @@ static inline zend_long php_add_var_hash(php_serialize_data_t data, zval *var) /
 		/* pass */
 	} else if (Z_TYPE_P(var) != IS_OBJECT) {
 		return 0;
-	} else if (Z_REFCOUNT_P(var) == 1 && (Z_OBJ_P(var)->properties == NULL || GC_REFCOUNT(Z_OBJ_P(var)->properties) == 1)) {
-		return 0;
 	}
 
 	/* References to objects are treated as if the reference didn't exist */