Plane: fix bug in RTL_AUTOLAND with rally points

[robertlong13]

After loading the rally point, ModeRTL:navigate checks if rally altitude has been reached before altitude_error_cm gets updated:

What this means is that with RTL_AUTOLAND = 1, if RTL is entered near a rally point, it will skip straight to the landing sequence instead of climbing to the rally point's altitude first.

Instead of the fix in this PR, is there a reason we don't update this variable inside of Plane::set_target_altitude_location? That would seem to be the better place to do it, but I wasn't 100% sure. There are multiple places where we set the altitude error variable immediately after this function:

ardupilot/ArduPlane/mode_guided.cpp

Lines 88 to 89 in 50eaa1c

	plane.set_target_altitude_location(temp);
	plane.altitude_error_cm = plane.calc_altitude_error_cm();

ardupilot/ArduPlane/mode_qrtl.cpp

Lines 199 to 200 in 5c3d464

	plane.set_target_altitude_location(loc);
	plane.altitude_error_cm = plane.calc_altitude_error_cm();

[tridge]

@robertlong13 can you provide a SITL log showing this issue? I'd like to see exactly how you have your params setup

[robertlong13]

00000007.zip

To reproduce:

1. Be within RTL radius of the rally point
2. Have achieved your target altitude for the currently executing waypoint and/or mode
3. Trigger the RTL manually from the GCS

[robertlong13]

Tested again today. This bug is still present, and this fix still works.
logs.zip

[IamPete1]

This looks OK to me. It will happen anyway at 10Hz in update_target_altitude so I don't think this will introduce any bugs, although it may make a existing one more likely.