pf: fix crash on low memory

pfr_create_kentry() can return NULL. Don't dereference the pointer it
returns until after we've checked it.

Fixes: 08ed87a4a276 ("pf: convert DIOCRSETADDRS to netlink")
See also: https://redmine.netgate.com/issues/23622
Sponsored by: Rubicon Communications, LLC ("Netgate")