OSD: assert() in share_map is replaced by if block
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.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>