libceph: allow ceph_buffer_put() to receive a NULL ceph_buffer

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/buffer.h

@@ -30,6 +30,7 @@ static inline struct ceph_buffer *ceph_buffer_get(struct ceph_buffer *b)
 
 static inline void ceph_buffer_put(struct ceph_buffer *b)
 {
+	if (b)
 		kref_put(&b->kref, ceph_buffer_release);
 }