Fix stackoverflow

Author: xBaank

Jint.Tests/Runtime/ArrayTests.cs

@@ -39,6 +39,14 @@ public void ArrayPrototypeToStringWithObject()
         Assert.Equal("[object Object]", result);
     }
 
+    [Fact]
+    public void ArrayPrototypeToStringSelfReference()
+    {
+        var result = _engine.Evaluate("Array.prototype.toString.call((c = [1, 2, 3, 4], c[1] = c, c))").AsString();
+
+        Assert.Equal("1,,3,4", result);
+    }
+
     [Fact]
     public void EmptyStringKey()
     {

Jint/Native/Array/ArrayPrototype.cs

@@ -1273,14 +1273,14 @@ private JsValue Join(JsValue thisObject, JsValue[] arguments)
             return JsString.Empty;
         }
 
-        static string StringFromJsValue(JsValue value)
+        static string StringFromJsValue(JsValue value, JsValue thisObject)
         {
-            return value.IsNullOrUndefined()
+            return value.IsNullOrUndefined() || thisObject == value
                 ? ""
                 : TypeConverter.ToString(value);
         }
 
-        var s = StringFromJsValue(o.Get(0));
+        var s = StringFromJsValue(o.Get(0), thisObject);
         if (len == 1)
         {
             return s;
@@ -1294,7 +1294,7 @@ static string StringFromJsValue(JsValue value)
             {
                 sb.Append(sep);
             }
-            sb.Append(StringFromJsValue(o.Get(k)));
+            sb.Append(StringFromJsValue(o.Get(k), thisObject));
         }
 
         return sb.ToString();