Fix debug message output when connecting to a logical slot.

Previously the message erroneously printed the same LSN twice as the
assignment to the start_lsn variable was before the message. Correct
that.

Reported-By: Marko Tiikkaja
Author: Marko Tiikkaja
Backpatch: 9.5, where logical decoding was introduced

src/backend/replication/logical/logical.c

@@ -406,11 +406,12 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 		 * decoding. Clients have to be able to do that to support synchronous
 		 * replication.
 		 */
-		start_lsn = slot->data.confirmed_flush;
 		elog(DEBUG1, "cannot stream from %X/%X, minimum is %X/%X, forwarding",
 			 (uint32) (start_lsn >> 32), (uint32) start_lsn,
 			 (uint32) (slot->data.confirmed_flush >> 32),
 			 (uint32) slot->data.confirmed_flush);
+
+		start_lsn = slot->data.confirmed_flush;
 	}
 
 	ctx = StartupDecodingContext(output_plugin_options,