Fix mavlogfile class for generic log files that have no timestamps.

[phollands]

I am updating MatrixPilot to use the latest mavlink generator for C and Python files. (it's been a while).

Before the enclosed commit, If I run mavlogdump.py on a generic matrixpilot raw log file I get the following errors message with previous MatrixPilot mavlink log files:-

Peters-iMac:tools phollands$ ./mavlogdump.py --dialect matrixpilot --no-timestamps /tmp/log.raw 
ERROR LOADING MAVNATIVE - falling back to python implementation
Traceback (most recent call last):
  File "./mavlogdump.py", line 113, in <module>
    m = mlog.recv_match(blocking=args.follow)
  File "/Users/phollands/MatrixPilot/Tools/MAVLink/mavlink/pymavlink/mavutil.py", line 344, in recv_match
    m = self.recv_msg()
  File "/Users/phollands/MatrixPilot/Tools/MAVLink/mavlink/pymavlink/mavutil.py", line 323, in recv_msg
    self.post_message(msg)
  File "/Users/phollands/MatrixPilot/Tools/MAVLink/mavlink/pymavlink/mavutil.py", line 1125, in post_message
    msg._link = self._link
AttributeError: 'mavlogfile' object has no attribute '_link'

And after initialising self._link as shown in the attached commit, I have no error, and I can process MatrixPilot log files again.

I think that function post_message will initialise self._link to 0 for the planner_format, but not for MatrixPilot :-). So here I initialise the _link to be 0, and anything that happens later on, can override that value of 0.

I'm looking forward to any comments that you might have.

Best wishes, Pete

[peterbarker]

@phollands Sorry, no comments.

Merged, thanks!