ceph: ensure auth ops are defined before use

In the create_authorizer method for both the mds and osd clients,
the auth_client->ops pointer is blindly dereferenced.  There is no
obvious guarantee that this pointer has been assigned.  And
furthermore, even if the ops pointer is non-null there is definitely
no guarantee that the create_authorizer or destroy_authorizer
methods are defined.

Add checks in both routines to make sure they are defined (non-null)
before use.  Add similar checks in a few other spots in these files
while we're at it.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>