[MERGE #5984 @MikeHolman] fix incorrect argument passed to itow for json syntax error

Merge pull request #5984 from MikeHolman:jsonerrorbuffer

Fixes #5975

Author: MikeHolman

lib/Runtime/Library/JSONScanner.h

@@ -27,7 +27,7 @@ namespace JSON
         void __declspec(noreturn) ThrowSyntaxError(int wErr)
         {
             char16 scanPos[16];
-            ::_itow_s(GetScanPosition(), scanPos, _countof(scanPos) / sizeof(char16), 10);
+            ::_itow_s(GetScanPosition(), scanPos, _countof(scanPos), 10);
             Js::JavascriptError::ThrowSyntaxError(scriptContext, wErr, scanPos);
         }
 

test/JSON/jsonerrorbuffer.js

@@ -0,0 +1,14 @@
+//-------------------------------------------------------------------------------------------------------
+// Copyright (C) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE.txt file in the project root for full license information.
+//-------------------------------------------------------------------------------------------------------
+
+let desiredLength = 10 * 1000 * 1000;
+let threw = false;
+const json = `"${'a'.repeat(desiredLength - 3)}",`;
+try {
+    JSON.parse(json);
+} catch(e) {
+    threw = true;
+}
+print(threw ? "Pass" : "Fail");

test/JSON/rlexe.xml

@@ -102,4 +102,9 @@
       <files>stackoverflow.js</files>
     </default>
   </test>
+  <test>
+    <default>
+      <files>jsonerrorbuffer.js</files>
+    </default>
+  </test>
 </regress-exe>