net80211: amrr_init: change order of commands

First run the KASSERT before trying to do the free operation.
Better for reporting and debugging.
Add a guard setting the value to NULL afterwards. NULL pointers
are a lot easier to test for.

Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43751

(cherry picked from commit cd9fee3d2c370e81109505c2abfd381af952e9b3)