Fix RBF checkpoint on reopen.

Previously, the `rbf.DB.opened` flag was set after `checkpoint()`
when reopening, however, this flag is checked by `checkpoint()` so
it was not properly executing.

rbf/db.go

@@ -129,6 +129,8 @@ func (db *DB) Open() (err error) {
 		return fmt.Errorf("create wal dir: %w", err)
 	}
 
+	db.opened = true
+
 	// Open write-ahead log & checkpoint to the end since no transactions are open.
 	if err := db.openWALSegments(); err != nil {
 		return fmt.Errorf("wal open: %w", err)
@@ -136,8 +138,6 @@ func (db *DB) Open() (err error) {
 		return fmt.Errorf("checkpoint: %w", err)
 	}
 
-	db.opened = true
-
 	return nil
 }