Fixes Oss-Fuzz issue: 21916 (#1180)

* Fix heap-buffer-overflow in json_reader
diff --git a/src/lib_json/json_reader.cpp b/src/lib_json/json_reader.cpp
index 2dca4ca..23cbe60 100644
--- a/src/lib_json/json_reader.cpp
+++ b/src/lib_json/json_reader.cpp
@@ -1270,7 +1270,7 @@
 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_;
     }