Fix memory leak in krb5_verify_init_creds

When copying most of the credentials from one cache to another in
copy_creds_except (called from get_vfy_cred, from
krb5_verify_init_creds), we need to free all of the credentials that
we read, not just the ones we copied.

[ghudson@mit.edu: edited commit message]

ticket: 7875 (new)
target_version: 1.12.2
tags: pullup