osd: tolerate sessionless con in fast dispatch path

We can now get a session cleared from a Connection at any time.  Change
the assert to an if in ms_fast_dispatch to cope.  It's pretty rare, but it
can happen, especially with delay injection.  In particular, a racing
thread can call mark_down() on us.

Fixes: #10209
Backport: giant
Signed-off-by: Sage Weil <sage@redhat.com>
Returning NULL here can lead to the OSD trying to send a map back along the Connection, which we're not allowed to do if we've marked them down.