MDEV-28855 SEGV around dict_free_vc_templ during DROP INDEX

protect shared dict_table_t::vc_templ with a mutex,
same as in ha_innobase::open

storage/innobase/handler/ha_innodb.cc

@@ -20988,7 +20988,6 @@ TABLE* innobase_init_vc_templ(dict_table_t* table)
 	DBUG_ENTER("innobase_init_vc_templ");
 
 	ut_ad(table->vc_templ == NULL);
-	table->vc_templ = UT_NEW_NOKEY(dict_vcol_templ_t());
 
 	TABLE	*mysql_table= innodb_find_table_for_vc(current_thd, table);
 
@@ -20998,6 +20997,7 @@ TABLE* innobase_init_vc_templ(dict_table_t* table)
 	}
 
 	mutex_enter(&dict_sys->mutex);
+	table->vc_templ = UT_NEW_NOKEY(dict_vcol_templ_t());
 	innobase_build_v_templ(mysql_table, table, table->vc_templ, NULL, true);
 	mutex_exit(&dict_sys->mutex);
 	DBUG_RETURN(mysql_table);