message: mavlink_ftp: fix size when reading files
  I'm not sure how this worked before.
diff: #include "crc32.h"
  #include "fs.h"
  +#include <algorithm>
  namespace mavsdk {
  @@ -539,7 +540,7 @@ void MavlinkFtp::_read()
  payload->session = _session;
  payload->opcode = _curr_op = CMD_READ_FILE;
  payload->offset = _bytes_transferred;
  - payload->size = 0;
  + payload->size = std::min(static_cast<uint32_t>(max_data_length), _file_size - _bytes_transferred);
  _send_mavlink_ftp_message(raw_payload);
  }
  