s3:messages: make the loop in msg_dgm_ref_recv() more robust against stale pointers

s3:messages: make the loop in msg_dgm_ref_recv() more robust against stale pointers

The interaction between msg_dgm_ref_recv() and msg_dgm_ref_destructor()
doesn't allow two references from messaging_dgm_ref() to be free'd
during the loop in msg_dgm_ref_recv().

In addition to the global 'refs' list, we also need to
have a global 'next_ref' pointer, which can be adjusted in
msg_dgm_ref_destructor().

As AD DC we hit this when using irpc in auth_winbind,
which uses imessaging_client_init().
In addition to the main messaging_dgm_ref() in smbd,
source3/auth/auth_samba4.c: prepare_gensec() and
make_auth4_context_s4() also generate a temporary
imessaging_context for auth_context->msg_ctx from within
auth_generic_prepare().

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13514

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>