MB-100: fixed minor compiler & dialyzer-found issues

Change-Id: I31aeeb8400e126a6b8685ba3e9214013bef9934d
Reviewed-on: http://review.couchbase.org/28065
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>