Batch command: avoid accessing potentially undefined context.principal
This might happen when the command is called directly in Python,
e.g. in installers and so on.
Pylint pylint-1.5.5-1.fc24.noarch caught this.
https://fedorahosted.org/freeipa/ticket/5838
Reviewed-By: Jan Cholasta <jcholast@redhat.com>