pf: add missing IPv6 length check

We failed to verify that the packet was long enough for the provided IPv6 packet
length. This could result in us walking off the end of the mbuf and panicing.

PR: 288224
Reported by: Robert Morris <rtm@lcs.mit.edu>
Tested by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: emaste
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D51324