Fixed compiler warning in cbsasl with msvc.

Change-Id: Ic62b38eedeb1c065492281f4938327c24bb593b1
Reviewed-on: http://review.couchbase.org/29766
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>