Objecter: set c->session to NULL if acting is empty

Otherwise, we might leave a session attached to the
CommandOp for an down OSD.  handle_osd_map will then
delete the session for the down OSD.  tick() will then
attempt to follow the invalid pointer to find a
connection over which to send a MPing.

Fixes: #5798
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>