ecl: rework optical flow selection logic

Fixes a race condition introduced by use of _is_dead-reckoning.
Only runs flow use logic when there is flow data available

ecl @ 62a15009

-Subproject commit 750e2eba2046d7fdb01e4b91be552156debbebdb
+Subproject commit 62a15009cf3debc577bfa100b2498799b7d95cde