"No SRTM data for this area" error in south pacific

#### Issue details
"No SRTM data for this area" error when I push "set home here" in south pacific :
https://discuss.ardupilot.org/t/no-srtm-data-for-this-area-on-south-pacific-sea/100457

#### Version
1.3.75, 1.3.80

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[V] Plane
[ ] Rover
[ ] Sub

#### Airframe type
VTOL

#### Hardware type
SITL, Pixhawk Cube

#### Logs
-

Start by reproducing the error in Mission Planner 1.3.75 or 1.3.80 with Plane SITL or a Pixhawk Cube, using “Set Home Here” in the south Pacific. Trace the SRTM lookup and determine why that area reports no data; done means setting home there completes without the reported error.

Written by the indexing model from the issue text.