Waypoint load fail to arduplane 3.8.5

#### Issue details
when loading wps to an arduplane w/ 3.8.5 load almost always fails on first load after establishing a connection unless a "read wps" is done first. Obviously, this clears the mission and you have to reload it before sending.

#### Version
1.3.62 -confirmed not an issue with 1.3.56

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

#### Airframe type
vtol_plane - octoquad

#### Hardware type
pixhawk 2.1


No source file, test, or entry point is named. Reproduce waypoint loading in Mission Planner 1.3.62 with ArduPlane 3.8.5 and Pixhawk 2.1, compare it with 1.3.56, and check the first load after connection versus loading after reading waypoints. Done means the initial waypoint load succeeds without first clearing and reloading the mission.

Written by the indexing model from the issue text.