libceph: potential NULL dereference in ceph_osdc_handle_map()

commit b72e19b9225d4297a18715b0998093d843d170fa upstream.

There are two places where we read "nr_maps" if both of them are set to
zero then we would hit a NULL dereference here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>