AP_Mission: get_item: ensure returned item is clean

[IamPete1]

Mission does not fill in all fields of the returned item, this means you can get random data. This results in getting bad data via misison ftp or scripting. The mavlink mission item protocol does not use this method. The other mission ftp types are OK because they always fill in the full set of information.

To reproduce, upload a misison and then read it back again. For example before.txt. Testing shown for SITL but the bug also exists on real hardware.

This is what we upload:

This is what we read back on master:

With the fix we get the same as what is written:

This only shows up in the x,y,z fields of a item without location because the other fields are defaulted here:

ardupilot/libraries/AP_Mission/AP_Mission.cpp

Lines 1574 to 1585 in 6efe210

	// command's position in mission list and mavlink id
	packet.seq = cmd.index;
	packet.command = cmd.id;
	// set defaults
	packet.current = 0; // 1 if we are passing back the mission command that is currently being executed
	packet.param1 = 0;
	packet.param2 = 0;
	packet.param3 = 0;
	packet.param4 = 0;
	packet.frame = 0;
	packet.autocontinue = 1;

A alternate (or complementary) fix would be to also set default values for the x,y and z values there.

[IamPete1]

I caved and added the defaulting in the mission_cmd_to_mavlink_int function too. There is no change in behavior from that fix because all callers pass in a clean command object.