fix race and parityshard leaks

fec.go

@@ -156,7 +156,7 @@ func (dec *fecDecoder) decode(in fecPacket) (recovered [][]byte) {
 					dlen := len(shards[k])
 					shards[k] = shards[k][:maxlen]
 					copy(shards[k][dlen:], dec.zeros)
-				} else {
+				} else if k < dec.dataShards {
 					shards[k] = xmitBuf.Get().([]byte)[:0]
 				}
 			}

sess.go

@@ -663,9 +663,10 @@ func (s *UDPSession) kcpInput(data []byte) {
 				if f.flag() == typeParity {
 					fecParityShards++
 				}
-				recovers := s.fecDecoder.decode(f)
 
+				// lock
 				s.mu.Lock()
+				recovers := s.fecDecoder.decode(f)
 				if f.flag() == typeData {
 					if ret := s.kcp.Input(data[fecHeaderSizePlus2:], true, s.ackNoDelay); ret != 0 {
 						kcpInErrors++