Fix panic on double Close of sqlite piece completion DB

storage/sqlite-piece-completion.go

@@ -64,6 +64,7 @@ func (me *sqlitePieceCompletion) Close() (err error) {
 	defer me.mu.Unlock()
 	if me.db != nil {
 		err = me.db.Close()
+		me.db = nil
 	}
 	return
 }