osd: don't assert if get_omap_iterator() returns NULL

Fixes: #4949
This can happen if the object does not exist and it's
a write operation. Just return -ENOENT.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #266 from ceph/wip-4949

osd: don't assert if get_omap_iterator() returns NULL

Reviewed-by: Sam Just <sam.just@inktank.com>