libpfct: ensure the initial allocation is large enough

Ensure that we allocate enough memory for the packed nvlist, no matter
what size hint was provided.

MFC after: 1 week
Reported by: R. Christian McDonald <rcm@rcm.sh>
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 6422599e74db4bb8b47cead46760d96601d8396a)