Fix for MDEV-8301; Statistics for a thread could be counted twice in SHOW STATUS while thread was ending

Fixed by adding a marker if we have added the thread statistics to the global counters.

sql/sql_class.h

@@ -1733,11 +1733,11 @@ public:
   /* Do not set socket timeouts for wait_timeout (used with threadpool) */
   bool skip_wait_timeout;
 
-  /* container for handler's private per-connection data */
-  Ha_data ha_data[MAX_HA];
-
   bool prepare_derived_at_open;
 
+  /* Set to 1 if status of this THD is already in global status */
+  bool status_in_global;
+
   /* 
     To signal that the tmp table to be created is created for materialized
     derived table or a view.
@@ -1746,6 +1746,9 @@ public:
 
   bool save_prep_leaf_list;
 
+  /* container for handler's private per-connection data */
+  Ha_data ha_data[MAX_HA];
+
 #ifndef MYSQL_CLIENT
   binlog_cache_mngr *  binlog_setup_trx_data();
 
@@ -3116,6 +3119,8 @@ public:
   {
     mysql_mutex_lock(&LOCK_status);
     add_to_status(&global_status_var, &status_var);
+    /* Mark that this THD status has already been added in global status */
+    status_in_global= 1;
     mysql_mutex_unlock(&LOCK_status);
   }