CVE-2017-13040/MPTCP: Clean up printing DSS suboption.

Do the length checking inline; that means we print stuff up to the point
at which we run out of option data.

First check to make sure we have at least 4 bytes of option, so we have
flags to check.

This fixes a buffer over-read discovered by Kim Gwan Yeong.

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