crush: fix stepping on unallocated memory
crush: fix stepping on unallocated memory

If size is 0 we can't write here.

Reported-by: pankaj singh <3f2c8fb344439fe3eb017bdb6b4dd96e933aa138@gmail.com>
Signed-off-by: Sage Weil <c5b6ff1fd4f0bde10db63d7a3e07166cf6c4229f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph