MB-10227 Avoid crash on allowedTimePeriod in compaction settings.

Change-Id: Ie6ca73585da93c8ae35efbbaea446a8fd2e8b0b3
Reviewed-on: http://review.couchbase.org/33647
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>