MB-8201 Don't update docs estimate if we can't grab it.

Arguably it's better to have our rough initial estimate for such
tap stream than to assume that nothing has to be transferred at all.

Change-Id: I8c06e879570b7ff4263e892bcd350e4af6da9bc4
Reviewed-on: http://review.couchbase.org/26166
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>