Fix the check to limit sync workers.

We don't allow to invoke more sync workers once we have reached the sync
worker limit per subscription. But the check to enforce this also doesn't
allow to launch an apply worker if it gets restarted.

This code was introduced by commit de43897122 but we caught the problem
only with the test added by recent commit c91f71b9dc which started failing
occasionally in the buildfarm.

As per buildfarm.
Diagnosed-by: Amit Kapila, Masahiko Sawada, Tomas Vondra
Author: Amit Kapila
Backpatch-through: 10
Discussion: https://postgr.es/m/CAH2L28vddB_NFdRVpuyRBJEBWjz4BSyTB=_ektNRH8NJ1jf95g@mail.gmail.com
	    https://postgr.es/m/f90d2b03-4462-ce95-a524-d91464e797c8@enterprisedb.com

src/backend/replication/logical/launcher.c

@@ -344,11 +344,11 @@ retry:
 	}
 
 	/*
-	 * If we reached the sync worker limit per subscription, just exit
+	 * We don't allow to invoke more sync workers once we have reached the sync
-	 * silently as we might get here because of an otherwise harmless race
+	 * worker limit per subscription. So, just return silently as we might get
-	 * condition.
+	 * here because of an otherwise harmless race condition.
 	 */
-	if (nsyncworkers >= max_sync_workers_per_subscription)
+	if (OidIsValid(relid) && nsyncworkers >= max_sync_workers_per_subscription)
 	{
 		LWLockRelease(LogicalRepWorkerLock);
 		return;