Fixed the failure of sp.test reported in the issue MDEV-86.

sql/sql_derived.cc

@@ -750,6 +750,7 @@ bool mysql_derived_optimize(THD *thd, LEX *lex, TABLE_LIST *derived)
     if (!derived->is_merged_derived())
     {
       JOIN *join= first_select->join;
+      unit->set_limit(first_select);
       unit->optimized= TRUE;
       if ((res= join->optimize()))
         goto err;