CVE-2017-12899/DECnet: Fix bounds checking.

If we're skipping over padding before the *real* flags, check whether
the real flags are in the captured data before fetching it.  This fixes
a buffer over-read discovered by Kamil Frankowicz.

Note one place where we don't need to do bounds checking as it's already
been done.

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