Fast session resumption segfault

[Alan DeKok]

Phil Mayers wrote:
> It's segfaulted a couple of times since then. The most recent occurrence
> was inside the "detail" module we run in post-auth, and I was able to
> examine the reply VPs - sure enough, the first VP in the list was a
> corrupted version of the Cached-Session-Reply VP; the ->next pointer and
> all the rest of the VPs were intact, but that one VP had corrupted
> payload, and an absurd length.
> 
> Weird stuff...

  I put in a hack to set the cached VPs to NULL when the session is
free'd.  Maybe that will help.

  Alan DeKok.