MB-9967 Don't crash when returning recovery task.

Change-Id: Iac67c83b224bba095b62b0bd9a01b754e04e0f2d
Reviewed-on: http://review.couchbase.org/32689
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>