Fix GH-20840: crash on nested object with var_dump().

ext/standard/tests/general_functions/gh20840.phpt

@@ -0,0 +1,38 @@
+--TEST--
+GH-20840 (var_dump() crash with nested objects)
+--CREDITS--
+bendrissou
+--SKIPIF--
+<?php
+if (ini_get('zend.max_allowed_stack_size') === false) {
+    die('skip No stack limit support');
+}
+if (getenv('SKIP_ASAN')) {
+    die('skip ASAN needs different stack limit setting due to more stack space usage');
+}
+?>
+--INI--
+zend.max_allowed_stack_size=512K
+--FILE--
+<?php
+class Node {
+    public $next;
+}
+
+$firstNode = new Node();
+$node = $firstNode;
+
+for ($i = 0; $i < 50000; $i++) {
+    $newNode = new Node();
+    $node->next = $newNode;
+    $node = $newNode;
+}
+
+var_dump($firstNode);
+
+while ($next = $firstNode->next) {
+    $firstNode->next = $next->next;
+}
+?>
+--EXPECTREGEX--
+^object\(Node\)#\d+ \(\d+\).*(nesting level too deep|["\s}]*)$

ext/standard/var.c

@@ -56,6 +56,12 @@ static void php_object_property_dump(zend_property_info *prop_info, zval *zv, ze
 {
 	const char *prop_name, *class_name;
 
+#ifdef ZEND_CHECK_STACK_LIMIT
+	if (UNEXPECTED(zend_call_stack_overflowed(EG(stack_limit)))) {
+		php_printf("%*cnesting level too deep", level + 1, ' ');
+		return;
+	}
+#endif
 	if (key == NULL) { /* numeric key */
 		php_printf("%*c[" ZEND_LONG_FMT "]=>\n", level + 1, ' ', index);
 	} else { /* string key */