MB-7830: Zero out key_stats buffer in observe logic

This will prevent us from sending garbage in some of the fields
if the key is not found on the server.

Change-Id: I561e95b46baee8f0994fe17b4df3e6c74ec3da4c
Reviewed-on: http://review.couchbase.org/24898
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>