libceph: fix ceph_msg_new error path

If memory allocation failed, calling ceph_msg_put() will cause GPF
since some of ceph_msg variables are not initialized first.

Fix Bug #970.

Signed-off-by: Henry C Chang <henry_c_chang@tcloudcomputing.com>
Signed-off-by: Sage Weil <sage@newdream.net>