mavproxy_ftp: Handle if no master when sending

[Ryanf55]

If you aren't connected, there is no master to send data to

MAV> ftp list APM
Listing APM
MAV> ERROR in command ['list', 'APM']: 'NoneType' object has no attribute 'mav'
Traceback (most recent call last):
  File "/home/ryan/Dev/ardu_ws/src/MAVProxy/MAVProxy/mavproxy.py", line 825, in process_stdin
    fn(args[1:])
  File "/home/ryan/Dev/ardu_ws/src/MAVProxy/MAVProxy/modules/mavproxy_ftp.py", line 159, in cmd_ftp
    self.cmd_list(args[1:])
  File "/home/ryan/Dev/ardu_ws/src/MAVProxy/MAVProxy/modules/mavproxy_ftp.py", line 238, in cmd_list
    self.send(op)
  File "/home/ryan/Dev/ardu_ws/src/MAVProxy/MAVProxy/modules/mavproxy_ftp.py", line 190, in send
    self.master.mav.file_transfer_protocol_send(self.network, self.target_system, self.target_component, payload)
AttributeError: 'NoneType' object has no attribute 'mav'