MDEV-26519 fixup: GCC 11 -Og -Wmaybe-uninitialized

GCC does not understand that the variable have_ndv determines
whether the variable ndv_ll is initialized. Let us add a
redundant initialization to pacify GCC.

sql/opt_histogram_json.cc

 /*
-   Copyright (c) 2021, MariaDB Corporation.
+   Copyright (c) 2021, 2022, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -611,7 +611,7 @@ int Histogram_json_hb::parse_bucket(json_engine_t *je, Field *field,
   bool have_ndv= false;
 
   double size_d;
-  longlong ndv_ll;
+  longlong ndv_ll= 0;
   StringBuffer<128> value_buf;
   int rc;