BUG/MAJOR: http-ana: fix private session retrieval on NTLM

BUG/MAJOR: http-ana: fix private session retrieval on NTLM

During the architectural review leading to commit 90b2154d93 ("MEDIUM:
muxes: always set conn->owner to the session that owns the connection"),
we wondered whether srv_conn->owner could ever be NULL or even invalid,
or if it ought to be changed to sess, and the projections of various use
cases, as well as a number of attempts to fool it led us to conclude
that it was always valid since the connection is private. So it was
considered safer not to start fiddling with the pointer in case it
could still match a previous session after a reuse, which would match
the scenario described in the session_add_conn() comment.

Actually there was exactly one case where a NULL could be met, and that
was covered by the preliminary call to conn_set_owner() that was removed
in that patch, precisely related to the one that the next patch tried to
address: in http-reuse always, after the second request on a connection
releases the connection, the owner can now become NULL, so if an NTLM
header is seen at this point, it will crash.

Interestingly, after the immediately following commit was merged,
d93c53b0df ("MEDIUM: session: always reset the conn->owner on backend
when installing mux"), the problem became immediate as the conn's
owner is now null during operation if the connection is not private, and
now the first response in NTLM is sufficient to crash the process. On
the other hand, thanks to the two patches above, we're now certain never
to meet a different session, which was the sought goal: either the session
is normal and it has no owner, or it's private and it has <sess> as owner.
Also with HTTP/1 (since the code explicitly checks for H1), there may be
a single request at a time on a connection so the owner should either be
the session or NULL.

So this patch finally implements the original plan, to pass <sess> to
session_add_conn(). The call is idempotent if the owner is already set,
but at least the function performs some preliminary sanity checks which
are quite welcome, so better continue to always call it.

Note that this is only for 3.4 or any branch that has exactly the two
patches above. And if the patches above were to ever be backported
(together), this one would be needed as well.

Thanks to Omkhar Arasaratnam for reporting this regression.