MB-8196 Fix badmatch in handle_call_via_servant.

Change-Id: I939ad9351e70e3d5ac99c4abfb9ae7f5e7c0f38a
Reviewed-on: http://review.couchbase.org/26081
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>