pf: Remove unneeded NULL check

pidx is never NULL, and is used unconditionally later on in the
function.
Add an assertion, as documentation for the requirement to provide an idx
pointer.

Reported by: clang --analyze
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 8cceacc0f1ee6a77c5f0566b8e6b0f054160fb20)