Further cleanup of ReorderBufferCommit().

On closer inspection, we can remove the "volatile" qualifier on
"using_subtxn" so long as we initialize that before the PG_TRY block,
which there's no particularly good reason not to do.
Also, push the "change" variable inside the PG_TRY so as to remove
all question of whether it needs "volatile", and remove useless
early initializations of "snapshow_now" and "using_subtxn".

src/backend/replication/logical/reorderbuffer.c

@@ -1258,12 +1258,10 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 					TimestampTz commit_time)
 {
 	ReorderBufferTXN *txn;
-	ReorderBufferIterTXNState *volatile iterstate = NULL;
-	ReorderBufferChange *change;
-
+	volatile Snapshot snapshot_now;
 	volatile CommandId command_id = FirstCommandId;
-	volatile Snapshot snapshot_now = NULL;
-	volatile bool using_subtxn = false;
+	bool		using_subtxn;
+	ReorderBufferIterTXNState *volatile iterstate = NULL;
 
 	txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
 								false);
@@ -1301,19 +1299,21 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 	/* setup the initial snapshot */
 	SetupHistoricSnapshot(snapshot_now, txn->tuplecid_hash);
 
+	/*
+	 * Decoding needs access to syscaches et al., which in turn use
+	 * heavyweight locks and such. Thus we need to have enough state around to
+	 * keep track of those.  The easiest way is to simply use a transaction
+	 * internally.  That also allows us to easily enforce that nothing writes
+	 * to the database by checking for xid assignments.
+	 *
+	 * When we're called via the SQL SRF there's already a transaction
+	 * started, so start an explicit subtransaction there.
+	 */
+	using_subtxn = IsTransactionOrTransactionBlock();
+
 	PG_TRY();
 	{
-		/*
-		 * Decoding needs access to syscaches et al., which in turn use
-		 * heavyweight locks and such. Thus we need to have enough state
-		 * around to keep track of those. The easiest way is to simply use a
-		 * transaction internally. That also allows us to easily enforce that
-		 * nothing writes to the database by checking for xid assignments.
-		 *
-		 * When we're called via the SQL SRF there's already a transaction
-		 * started, so start an explicit subtransaction there.
-		 */
-		using_subtxn = IsTransactionOrTransactionBlock();
+		ReorderBufferChange *change;
 
 		if (using_subtxn)
 			BeginInternalSubTransaction("replay");
@@ -1323,7 +1323,7 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 		rb->begin(rb, txn);
 
 		iterstate = ReorderBufferIterTXNInit(rb, txn);
-		while ((change = ReorderBufferIterTXNNext(rb, iterstate)))
+		while ((change = ReorderBufferIterTXNNext(rb, iterstate)) != NULL)
 		{
 			Relation	relation = NULL;
 			Oid			reloid;