Adjust initial distance when destination is changed for obstacle check (#6860)

Changed destination may create a situation when the distance actor moved between 2 update calls is less than initial distance because destination has been changed. This forces actor to take evasive action when there is no actual obstacle.

I've added a change log record for 0.49 but the fix is quite simple and could be backported to 0.48.

Fixes #6860 (closed).