Add missing initialization

Add missing initialization

The backend running REPACK can check DecodingWorkerShared->initialized
before the worker could have the chance to initialize it, possibly
leading to wrong behavior.

While at it, remove DecodingWorkerShared->worker_dsm_segment, because
that doesn't actually need to be in shared memory; a simple local-memory
global variable is enough.

Oversights in commit 28d534e2ae0a.

Author: Antonin Houska <ah@cybertec.at>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/18181295-8375-4789-ad32-269d78d6001e@gmail.com