MultiVehicle support - one vehicle missing from drop-down

While connecting the mission planner to an antenna tracker (which was in turn connected to a quadcopter), the vehicle selection drop-down only showed the tracker. See pic below which shows both vehicles were visible on the map.

[tlog_tracker_and_quad.zip](https://github.com/ArduPilot/MissionPlanner/files/260577/tlog_tracker_and_quad.zip)

Reproduce the issue in Mission Planner using the attached tlog_tracker_and_quad.zip while connecting the mission planner to an antenna tracker and quadcopter. Compare the vehicle selection drop-down with the map, where both vehicles are visible. Done means the drop-down includes both the tracker and the quadcopter.

Written by the indexing model from the issue text.