databases/cego: update 2.38.16 -> 2.38.18

• Fix in CegoTableManager::createForeignKey, for empty tables, referenced attributes names have not been checked. This might lead to invalid key objects.
• Fix in CegoSelect::prepare, expression alias in select list are checked now for union selects. Alias definition in select expression list should be identical for all select statements in union

Submitted by: Bjoern Lemke <lemke@lemke-it.com>