CVE-2017-13689/IKEv1: Fix addr+subnet length check.

An IPv6 address plus subnet mask is 32 bytes, not 20 bytes.
16 bytes of IPv6 address, 16 bytes of subnet mask.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

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