pf: ignore state update with invalid rtableid

It's possible for a peer to send us a state update with an rtableid we don't
support (i.e. >= net.fibs).
Drop these updates rather than potentially crashing later by setting an invalid
fib number.

PR: 287981
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 4af4fefedd1bf14b5a27387075edde98744b722e)