MDEV-23542 Server crashes in thd_clear_errors()

1) Checked presence of mysys_var.
2) Removed unneeded thd_clear_errors,
  its content added to THD::change_user.

sql/sql_class.cc

@@ -299,17 +299,6 @@ THD *thd_get_current_thd()
   return current_thd;
 }
 
-/**
-  Clear errors from the previous THD
-
-  @param thd              THD object
-*/
-void thd_clear_errors(THD *thd)
-{
-  my_errno= 0;
-  thd->mysys_var->abort= 0;
-}
-
 
 /**
   Get thread attributes for connection threads
@@ -1422,7 +1411,10 @@ void THD::change_user(void)
     cleanup();
   cleanup_done= 0;
   reset_killed();
-  thd_clear_errors(this);
+  /* Clear errors from the previous THD */
+  my_errno= 0;
+  if (mysys_var)
+    mysys_var->abort= 0;
 
   /* Clear warnings. */
   if (!get_stmt_da()->is_warning_info_empty())