MB-7629: Small improvement for view reductions.c

Change-Id: If718a76fb7c44cb7ba7ccfda9bff6ef31a1f89f8
Reviewed-on: http://review.couchbase.org/25619
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>