libceph: fix crypto key null deref, memory leak

Avoid crashing if the crypto key payload was NULL, as when it was not correctly
allocated and initialized.  Also, avoid leaking it.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
diff --git a/net/ceph/crypto.h b/net/ceph/crypto.h
index 1919d15..3572dc5 100644
--- a/net/ceph/crypto.h
+++ b/net/ceph/crypto.h
@@ -16,7 +16,8 @@
 
 static inline void ceph_crypto_key_destroy(struct ceph_crypto_key *key)
 {
-	kfree(key->key);
+	if (key)
+		kfree(key->key);
 }
 
 extern int ceph_crypto_key_clone(struct ceph_crypto_key *dst,