libpfctl: handle allocation failure

While it's unlikely for userspace to fail to allocate memory it is still
possible. Handle malloc() returning NULL.

Reported by: Bill Meeks <bill@themeeks.net>
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 33d55d0d0f33787e9e2796b5000be73af42573bc)