mds: fix crash killing sessions without conn

Bug was introduced in 54eca56695  (mds: use Connection::mark_down()).
Used to call SimpleMessenger::mark_down, which had a check for
connection==NULL, when calling conn->mark_down() we must
now do our own nullness check.

Fixes: #9173

Signed-off-by: John Spray <john.spray@redhat.com>