Fix assertion failure in apply worker.

During exit, the logical replication apply worker tries to release session
level locks, if any. However, if the apply worker exits due to an error
before its connection is initialized, trying to release locks can lead to
assertion failure. The locks will be acquired once the worker is
initialized, so we don't need to release them till the worker
initialization is complete.

Reported-by: Alexander Lakhin
Author: Hou Zhijie based on inputs from Sawada Masahiko and Amit Kapila
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/2185d65f-5aae-3efa-c48f-fb42b173ef5c@gmail.com

src/backend/replication/logical/worker.c

@@ -331,6 +331,9 @@ static TransactionId stream_xid = InvalidTransactionId;
  */
 static uint32 parallel_stream_nchanges = 0;
 
+/* Are we initializing a apply worker? */
+bool		InitializingApplyWorker = false;
+
 /*
  * We enable skipping all data modification changes (INSERT, UPDATE, etc.) for
  * the subscription if the remote transaction's finish LSN matches the subskiplsn.
@@ -4526,6 +4529,8 @@ ApplyWorkerMain(Datum main_arg)
 	WalRcvStreamOptions options;
 	int			server_version;
 
+	InitializingApplyWorker = true;
+
 	/* Attach to slot */
 	logicalrep_worker_attach(worker_slot);
 
@@ -4548,6 +4553,8 @@ ApplyWorkerMain(Datum main_arg)
 
 	InitializeApplyWorker();
 
+	InitializingApplyWorker = false;
+
 	/* Connect to the origin and start the replication. */
 	elog(DEBUG1, "connecting to publisher using connection string \"%s\"",
 		 MySubscription->conninfo);