CVE-2017-13000/IEEE 802.15.4: Fix bug introduced two fixes prior.

We've already advanced the pointer past the PAN ID, if present; it now
points to the address, so don't add 2 to it.

This fixes a buffer over-read discovered by Forcepoint's security
researchers Otto Airamo & Antti Levomäki.

Add a test using the capture file supplied by the reporter(s).