Simplify read_guts.

Change-Id: I6232d1ee1c50ff9767ae357aea434bb774587206
Reviewed-on: http://review.couchbase.org/38987
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>