Fix Failed JSON stringify with NaN

http://www.ecma-international.org/ecma-262/5.1/#sec-15.12.3
Depending to Note4, NaN is reprensented as the String null

JerryScript-DCO-1.0-Signed-off-by: pius lee idkiller@gmail.com

tests/jerry/regression-test-issue-741.js

@@ -0,0 +1,25 @@
+// Copyright 2015 Samsung Electronics Co., Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+var a = JSON.stringify (b=+'�');
+
+assert(a === "null");
+
+var b = JSON.stringify (b=-'�0001');
+
+assert(b === "null");
+
+var c = JSON.stringify (b=+'�');
+
+assert(c === "null");