MB-5307: dont crash doctor if somebody asks about unknown node

Spotted that useless crash in diags of MB-7865. AFAIK testrunner is
quite a user of /nodes/self API which is using that and looks like at
that time doctor did not yet have it's own status.

Change-Id: Ic01f47f088f1d8a7c04b603f80f1fa72bc511317
Reviewed-on: http://review.couchbase.org/25003
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>