fix a potential unexpected closing while bucket has exhausted

session.go

@@ -392,8 +392,12 @@ func (s *Session) keepalive() {
 			s.notifyBucket() // force a signal to the recvLoop
 		case <-tickerTimeout.C:
 			if !atomic.CompareAndSwapInt32(&s.dataReady, 1, 0) {
-				s.Close()
-				return
+				// recvLoop main block on bucket == 0, in this case,
+				// session should not be closed.
+				if atomic.LoadInt32(&s.bucket) > 0 {
+					s.Close()
+					return
+				}
 			}
 		case <-s.die:
 			return