MAVProxy sending invalid terrain data above 60N

I think that MAVProxy is sending invalid terrain data to the vehicle for latitudes above 60N for which the ardupilot SRTM database has no data. This means that a user in finland (and many other places probably) could attempt to fly a terrain altitude mission and all would appear fine until the vehicle crashed into the ground.

This can be reproduced in SITL by doing the following:

- edit the Tools/autotest/Locations.txt file and add finland as "62.23075, 25.6993,0,0"
- ../Tools/autotest/sim_vehicle.py -L finland --map --console
- create a short mission with a takeoff, waypoint (with an altitude of 10m AGL), RTL
- fly the mission

The vehicle will fly the mission just as if it's a relative altitude. I.e. it will not climb over mountains.

Here is the issue on the vehicle side: https://github.com/ArduPilot/ardupilot/issues/12923

Add Finland to Tools/autotest/Locations.txt, run ../Tools/autotest/sim_vehicle.py -L finland --map --console, and reproduce the short terrain-altitude mission described in the issue. Trace how MAVProxy sends terrain data when the SRTM database has no coverage above 60N; done means the mission no longer behaves like relative altitude and terrain following is safe in the reproduction.

Written by the indexing model from the issue text.