Wireshark showing 0 for negative int32

[ThomasDebrunner]

With mission items with negative y (negative int32 value), the wireshark plugin incorrectly shows 0, even though the data is clearly not 0 (as seen in the hex dump)

[hamishwillee]

@peterbarker Can you look at this please. I cross posted to the generator repo ArduPilot/pymavlink#701

[hamishwillee]

@ThomasDebrunner Request for a test file here: ArduPilot/pymavlink#701 (comment)

[hamishwillee]

@ThomasDebrunner - if you want this looked at we need test file please.