Account for keylength while allocating buffer for allKeys

While increasing the buffersize (when needed) when the
AllKeysAPI is invoked, we will need to account for two
additional bytes while allocating the key, for its length.

Change-Id: Iba68c4ae7bccae20f97d4e98350d5105093c3487
Reviewed-on: http://review.couchbase.org/41595
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>