Valgrind error in AP_Mission_ChangeDetector

[rmackay9]

During the implementation of ARC Waypoints (PR: #31139) we found that Valgrind complains about AP_Mission_ChangeDetector using uninitialised variables.

We have a PR here that includes an autotest that fails due to the issue #31492

I suspect the issue is actually very rare and innocuous but we should still investigate and fix it