libcephfs: fix ceph_get_osd_crush_location

(a) Allow NULL buffer when length is zero to get the target buffer
length. (b) fix edge case where buffer length is the exact size
required.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #132 from ceph/wip-libcephfs-fix

libcephfs: fix ceph_get_osd_crush_location

Reviewed-by: Sage Weil <sage@inktank.com>