The bson_as_json function has a shortcut for an empty object, which has this code:

   if (bson_empty0 (bson)) {
      if (length) {
         *length = 2;
      }

      return bson_strdup ("{ }");
   }

https://github.com/mongodb/libbson/blob/master/src/bson/bson.c#L2672

As you can see, the returned string is three characters long, but we only return 2 as the output length. It should be 3.