libpfctl: memory leak fix

We must remember to free the nvlist we create from the kernel's response
to DIOCGETSTATESNV, on every iteration.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30957

(cherry picked from commit 0e9f1892ec739d7fbd854af699507167a0a5dde2)