pf: explicitly NULL state key pointers

After the pf_state_insert() call we may not use these pointers again.
Explicitly NULL them to ensure we don't.
Also NULL them out if we free the keys directly.

Reviewed by: glebius, markj
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D49553

(cherry picked from commit 592418343348bcf4355c249f53cff89ed90ea1f5)