releasing ParallelApplyTxnHash when pa_launch_parallel_worker returns NULL

[Ted Yu]

Hi,
I was reading src/backend/replication/logical/applyparallelworker.c .
In `pa_allocate_worker`, when pa_launch_parallel_worker returns NULL, I
think the `ParallelApplyTxnHash` should be released.

Please see the patch.

Thanks