kdb_db2's ctx_iterate makes an convenience alias to dbc->db in order
to call more invoke call the DB's seq method. This alias may become
invalidated if the callback writes to the DB, since ctx_lock() may
re-open the DB in order to acquire a write lock. Fix the bug by
getting rid of the convenience alias.
Most KDB iteration operations in the code base do not write to the DB,
but kdb5_util update_princ_encryption does.
Bug discovered and diagnosed by firstname.lastname@example.org.
Commit By: ghudson