OSD: adjust share_map() to handle the case that the osd is down

The assert was hitting while OSd is waiting for becoming healthy
in handle_osd_map(). This can happen while io is going on and
OSDs are made down forcefully by say osd thrash command.
So, the fix could be instead of asserting just return from here.

Fixes: http://tracker.ceph.com/issues/8646

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>