pf: when extracting an IPv6 address, make sure we got an IPv6 address

Our code assumes that when we're configured to get IPv6 addresses
out of a TRANS_PF transparent proxy connection, we actually will.
But we didn't check that, and so FreeBSD started warning us about a
potential NULL pointer dereference.

Fixes part of bug 31687; bugfix on 0.2.3.4-alpha when this code was
added.