Copter/SITL: Guided mode can produce, "Should not be called on invalid location"

[rmackay9]

It is possible to make Copter running in SITL produce the, "PANIC: Should not be called on invalid location".

The steps are:

1. enable GPS and optical flow
   • param set EK3_SRC2_VELXY 5
   • parm load ../Tools/autotest/default_params/copter-optflow.parm
2. param set SIM_GPS_DISABLE 1
3. GUIDED
4. arm throttle force
5. takeoff 20
6. right-mouse-button click on the SITL map and select "Fly To" and input any location

I suspect this is related to SIM_GPS_DISABLE=1 not completely disabling the GPS. The vehicle appears on the map (it does not when GPS1_TYPE=0) and this may allow the EKF origin (and home?) to be set. Perhaps we're missing a check in the compulsory arming checks?