MB-5307: avoid parsing value that we already know

get_zero_open_checkpoint_vbuckets code was preparing map from stat
names to vbuckets, and than matches stat name, but instead of grabbing
corresponding vbucket from map, it was parsing stat name that's we
just found equal to what we've built.

Change-Id: I826e72fd88a60c3ec365b54bd7cab9d5f69c2a10
Reviewed-on: http://review.couchbase.org/23595
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>