Fix heap-buffer-overflow in json_reader

Author: kabeer27

Diff for: src/lib_json/json_reader.cpp

@@ -1270,7 +1270,7 @@ void OurReader::skipSpaces() {
 void OurReader::skipBom(bool skipBom) {
   // The default behavior is to skip BOM.
   if (skipBom) {
-    if (strncmp(begin_, "\xEF\xBB\xBF", 3) == 0) {
+    if ((end_ - begin_) >= 3 && strncmp(begin_, "\xEF\xBB\xBF", 3) == 0) {
       begin_ += 3;
       current_ = begin_;
     }