Copter: check for fence breaches when disarmed

[chobitsfan]

It fix the problem that copters keep showing "invalid Fence Boundaries", even user have upload a valid polygon fence. This problem makes copters can not use polygon fence

#7411 add a pre-arm check for polygon fence validity. But in copter, polygon fence is not loaded before armed and leave _boundary_valid false. Which cause polygon fence per-arm check fail and make copter can not be armed. It create deadlock and makes copter unable to use polygon fence

[OXINARF]

I think this isn't the correct way to deal with it. I think we should change

ardupilot/ArduCopter/fence.cpp

Lines 11 to 14 in 8108931

	// ignore any fence activity when not armed
	if(!motors->armed()) {
	return;
	}

to be after

ardupilot/ArduCopter/fence.cpp

Lines 18 to 19 in 8108931

	// check for new breaches; new_breaches is bitmask of fence types breached
	const uint8_t new_breaches = fence.check();

So that we still don't do anything when disarmed, but we do check for fence breaches.
Main reason is that the fence pre-arm check actually checks if any fence has been breached - that's not ever going to be true if we don't call check on AP_Fence while disarmed.

[peterbarker]

I have a feeling this stuff might be the cause of a weird behaviour you get on Copter sometimes where a vehicle arms, failsafes and instantly disarms, but you can then rearm no problems and the vehicle remains armed.

Again, no hard data, but it's possible @OXINARF 's suggestion might fix that problem.

[chobitsfan]

OK, I have tested @OXINARF 's changes on both SITL and my quadcopter. It looks good. Thank you very much

[OXINARF]

@chobitsfan Great! Can you just change the commit message? Something like Copter: check for fence breaches when disarmed better describes what it does.

[khancyr]

nitpick : miss a space !

[chobitsfan]

ok

[rmackay9]

Merged, thanks very much! This will be included in Copter-3.6.0-rc3!