Fix bug #6315 smbd crashes doing vfs_full_audit on IPC$ close event.

Fix bug #6315 smbd crashes doing vfs_full_audit on IPC$ close event.
The underlying problem
is that once SMBulogoff is called, all server_info contexts associated with the
vuid should become invalid, even if that's the context being currently used by
the connection struct (tid). When the SMBtdis comes in it doesn't need a valid
vuid value, but the code called inside vfs_full_audit always assumes that there
is one (and hence a valid conn->server_info pointer) available.

This is actually a bug inside the vfs_full_audit and other code inside Samba,
which should only indirect conn->server_info on calls which require AS_USER to
be set in our process table. I could fix all these issues, but there's no
guarentee that someone might not add more code that fails this assumption, as
it's a hard assumption to break (it's usually true).

So what I've done is to ensure that on SMBulogoff the previously used
conn->server_info struct is kept around to be used for print debugging purposes
(it won't be used to change to an invalid user context, as such calls need
AS_USER set). This isn't strictly correct, as there's no association with the
(now invalid) context being freed and the call that causes conn->server_info to
be indirected, but it's good enough for most cases.

The hard part was to ensure that once a valid context is used again (via new
sessionsetupX calls, or new calls on a still valid vuid on this tid) that we
don't leak memory by simply replacing the stored conn->server_info pointer. We
would never actually leak the memory (as all conn->server_info pointers are
talloc children of conn), but with the previous patch a malicious client could
cause many server_info structs to be talloced by the right combination of SMB
calls. This new patch introduces free_conn_server_info_if_unused(), which
protects against the above.
Jeremy.