CVE-2017-13009/IPv6 mobility: Add a bounds check.

This fixes a buffer over-read discovered by Brian 'geeknik' Carpenter.

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

While we're at it:

Add a comment giving the RFC for IPv6 mobility headers.

Clean up some bounds checks to make it clearer what they're checking, by
matching the subsequent EXTRACT_ calls or memcpy.

For the binding update, if none of the flag bits are set, don't check
the individual flag bits.