libceph: avoid NULL kref_put when osd reset races with alloc_msg

The ceph_on_in_msg_alloc() method drops con->mutex while it allocates a
message.  If that races with a timeout that resends a zillion messages and
resets the connection, and the ->alloc_msg() method returns a NULL message,
it will call ceph_msg_put(NULL) and BUG.

Fix by only calling put if msg is non-NULL.

Fixes http://tracker.newdream.net/issues/3142

Signed-off-by: Sage Weil <sage@inktank.com>