Check for valid assignee before reassigning.

* jeepyb/cmd/update_bug.py(set_in_progress): Check that the value
returned by getByOpenIDIdentifier is usable before trying to
reassign the bug.

Change-Id: Idbc03fdc0663e83d96292171bd93f5e9f4fe1adb
Reviewed-on: https://review.openstack.org/19497
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Dolph Mathews <dolph.mathews@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins