MB-10976 Fix crash on GET /settings/maxParallelIndexers.

Change-Id: I635fb05fd5bf38ca1bc39826d09b78b44d1c2240
Reviewed-on: http://review.couchbase.org/36421
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>