Fix handling of empty constructor names (#2922)

Co-authored-by: James Newton-King <james@newtonking.com>

Src/Newtonsoft.Json.Tests/JsonConvertTest.cs

@@ -118,6 +118,17 @@ namespace Newtonsoft.Json.Tests
             }, "No JSON content found. Path '', line 0, position 0.");
         }
 
+        [Test]
+        public void NoConstructorName() {
+            ExceptionAssert.Throws<JsonException>(
+                () => JsonConvert.DeserializeObject("[new \0("),
+                "Empty constructor name. Path '', line 1, position 6.");
+
+            ExceptionAssert.Throws<JsonException>(
+                () => JsonConvert.DeserializeObject("{'x':new \0("),
+                "Empty constructor name. Path 'x', line 1, position 10.");
+        }
+
         [Test]
         public void PopulateObjectWithOnlyComment()
         {
@@ -1830,4 +1841,4 @@ namespace Newtonsoft.Json.Tests
             public DateTime Expiration { get; set; }
         }
     }
-}
+}

Src/Newtonsoft.Json/JsonTextReader.cs

@@ -1937,6 +1937,11 @@ namespace Newtonsoft.Json
                         throw JsonReaderException.Create(this, "Unexpected character while parsing constructor: {0}.".FormatWith(CultureInfo.InvariantCulture, currentChar));
                     }
                 }
+                
+                if (initialPosition == endPosition)
+                {
+                    throw JsonReaderException.Create(this, "Empty constructor name.");
+                }
 
                 _stringReference = new StringReference(_chars, initialPosition, endPosition - initialPosition);
                 string constructorName = _stringReference.ToString();