Reset monster plans if monster has been shifted away from start of plan path.

Fixes #7592 
To reproduce, go somewhere dark, give yourself akido and melee/unarmed 9, spawn a crawler, and wait.  You might have to go through a few crawlers before it happens, but it's pretty consistent.
What happens is you counter-throw the crawler out of sight range, which means the next time it acts, it can't see you and tries to navigate to your last known position.  Since it was next to you the last time it acted, it still thinks it's next to you and just attacks.
Thanks Coolthulhu for the details needed to reproduce the issue.