mavutil: Do not use unknown messages to calculate number of lost packets

[joshanne]

Found this one when adding a custom mavlink message.
The decoding of the packet affects the mav_loss calculation because the unknown message does not have its header populated.

To a user, this could appear as though the vehicle radio link is intermittent/bad.

[nexton-winjeel]

Just to clarify what @joshanne said:

• Previously, if a message is unknown in pymavlink, it gets constructed into a dummy message with the sequence number (seq) set to 0.
• The number of lost messages is calculated as the difference between the sequence number of the previous message and the sequence number of the current message.
• With the unknown message having a sequence number always set to 0, the number of lost messages is incremented by an average of 128 for every unknown message received.

With this change, the number of lost messages will increment by 1 for each unknown message received.

[tridge]

good find, thanks!