MDEV-18225 Avoid use of LOCK_prepared_stmt_count mutex in Statement_map destructo

This mutex can be freed when server shuts down (when thread_count goes down to 0)
, but it is still used inside THD::~THD() when Statement_map is destroyed.

The fix is to call Statement_map::reset() at the point where thread_count
is still positive, and avoid locking LOCK_prepared_stmt_count in THD
destructor.

sql/sql_class.cc

@@ -1518,6 +1518,7 @@ void THD::cleanup(void)
   sp_cache_clear(&sp_func_cache);
 
   mysql_ull_cleanup(this);
+  stmt_map.reset();
   /* All metadata locks must have been released by now. */
   DBUG_ASSERT(!mdl_context.has_locks());
 
@@ -3777,11 +3778,13 @@ void Statement_map::erase(Statement *statement)
 void Statement_map::reset()
 {
   /* Must be first, hash_free will reset st_hash.records */
+  if (st_hash.records)
+  {
     mysql_mutex_lock(&LOCK_prepared_stmt_count);
     DBUG_ASSERT(prepared_stmt_count >= st_hash.records);
     prepared_stmt_count-= st_hash.records;
     mysql_mutex_unlock(&LOCK_prepared_stmt_count);
-
+  }
   my_hash_reset(&names_hash);
   my_hash_reset(&st_hash);
   last_found_statement= 0;
@@ -3790,12 +3793,8 @@ void Statement_map::reset()
 
 Statement_map::~Statement_map()
 {
-  /* Must go first, hash_free will reset st_hash.records */
-  mysql_mutex_lock(&LOCK_prepared_stmt_count);
-  DBUG_ASSERT(prepared_stmt_count >= st_hash.records);
-  prepared_stmt_count-= st_hash.records;
-  mysql_mutex_unlock(&LOCK_prepared_stmt_count);
-
+  /* Statement_map::reset() should be called prior to destructor. */
+  DBUG_ASSERT(!st_hash.records);
   my_hash_free(&names_hash);
   my_hash_free(&st_hash);
 }