CCBC-188 Check if SASL struct valid before disposing

Change-Id: I218360b4b17e0e2a3abe621343531cd9502ef7f7
Reviewed-on: http://review.couchbase.org/25851
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>