MB-100: Fix some warnings

Some additional warnings are shown when compiled with -Wextra
and When compiled clang. This commit fixes those.

Change-Id: I8f45fc4e5ab57fce19c2b21ac76939940fe7701b
Reviewed-on: http://review.couchbase.org/27655
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>