Fix parallel sort, broken by the balanced merge patch.

Fix parallel sort, broken by the balanced merge patch.

The code for initializing the tapes on each merge iteration was skipped
in a parallel worker. I put the !WORKER(state) check in wrong place while
rebasing the patch.

That caused failures in the index build in 'multiple-row-versions'
isolation test, in multiple buildfarm members. On my laptop it was easier
to reproduce by building an index on a larger table, so that you got a
parallel sort more reliably.