MDEV-13693 : Fix parameter passing to my_error.

char* parameter is expected by the message ER_KEY_COLUMN_DOES_NOT_EXITS,
thus pass char*, rather than LEX_STRING.

sql/sql_class.cc

@@ -230,7 +230,7 @@ bool Foreign_key::validate(List<Create_field> &table_fields)
                          sql_field->field_name)) {}
     if (!sql_field)
     {
-      my_error(ER_KEY_COLUMN_DOES_NOT_EXITS, MYF(0), column->field_name);
+      my_error(ER_KEY_COLUMN_DOES_NOT_EXITS, MYF(0), column->field_name.str);
       DBUG_RETURN(TRUE);
     }
     if (type == Key::FOREIGN_KEY && sql_field->vcol_info)