Fix KDB iteration when callback does write calls

Fix KDB iteration when callback does write calls

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 will.fiveash@oracle.com.

ticket: 7096
target_version: 1.10.1
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25723 dc483132-0cff-0310-8789-dd5450dbe970