vendor: update github.com/pkg/sftp because dep insisted

vendor/github.com/pkg/sftp/server.go

@@ -134,7 +134,7 @@ func (svr *Server) sftpServerWorker(pktChan chan requestPacket) error {
 		case *sshFxpOpenPacket:
 			readonly = pkt.readonly()
 		case *sshFxpExtendedPacket:
-			readonly = pkt.SpecificPacket.readonly()
+			readonly = pkt.readonly()
 		}
 
 		// If server is operating read-only and a write operation is requested,
@@ -304,9 +304,18 @@ func (svr *Server) Serve() error {
 
 		pkt, err = makePacket(rxPacket{fxp(pktType), pktBytes})
 		if err != nil {
-			debug("makePacket err: %v", err)
-			svr.conn.Close() // shuts down recvPacket
-			break
+			switch errors.Cause(err) {
+			case errUnknownExtendedPacket:
+				if err := svr.serverConn.sendError(pkt, ErrSshFxOpUnsupported); err != nil {
+					debug("failed to send err packet: %v", err)
+					svr.conn.Close() // shuts down recvPacket
+					break
+				}
+			default:
+				debug("makePacket err: %v", err)
+				svr.conn.Close() // shuts down recvPacket
+				break
+			}
 		}
 
 		pktChan <- pkt