MDEV-19351 statistics_for_command_is_needed: Conditional jump or move

           depends on uninitialised value

Initialized THD::force_read_stats introduced in the patch for MDEV-17605.
Leaving this field uninitialized in the constructor of the THD class may
trigger reading statistical data that is not needed.

sql/sql_class.cc

@@ -983,6 +983,7 @@ THD::THD(bool is_wsrep_applier)
   memset(&invoker_host, 0, sizeof(invoker_host));
   prepare_derived_at_open= FALSE;
   create_tmp_table_for_derived= FALSE;
+  force_read_stats= FALSE;
   save_prep_leaf_list= FALSE;
   /* Restore THR_THD */
   set_current_thd(old_THR_THD);