Fix minor static analysis defects

Fix minor static analysis defects

Remove an unused variable in krb5_ldap_create().  Handle the return
value from krb5_dbe_get_string() in the certauth test plugin module.
Handle the return value from k5_expand_path_tokens() in
k5_rc_default().  Remove dead assignments in
krb5_get_credentials_for_user() and kg_accept_krb5().

[ghudson@mit.edu: squashed and edited commit message; simplified
k5_rc_default() change]