db_auc.c: verify hex key sizes read from DB

Replace commented out size check for Ki with a real check, and use it
consistently for Ki, K, OP and OPC. Add a test that sets all keys to the
wrong size and tries to read them.

Related: OS#2565
Change-Id: Ib8e8e9394fb65c6e7932ce9f8bebc321b99f7696
3 files changed