pgsql: 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(at)cybertec(dot)at>
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Discussion: https://postgr.es/m/18181295-8375-4789-ad32-269d78d6001e@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/05c401d5786a05ea630e884ffa492aa01683d15b

Modified Files
--------------
src/backend/commands/repack.c | 1 +
src/backend/commands/repack_worker.c | 7 +++++--
src/include/commands/repack_internal.h | 1 -
3 files changed, 6 insertions(+), 3 deletions(-)