Refactor krb5_vset_error_message() to remove the many conditional
tests on the existence of krb5_context. If there is no krb5_context
then _krb5_debug() is a no-op. Therefore, there is no point in
performing any of the other work.
Change-Id: Ib88b592a542a195f27e352a80ced0a98a6f85300