libceph: don't call crypto_free_sync_skcipher() on a NULL tfm

In set_secret(), key->tfm is assigned to NULL on line 55, and then
ceph_crypto_key_destroy(key) is executed.

ceph_crypto_key_destroy(key)
crypto_free_sync_skcipher(key->tfm)
crypto_free_skcipher(&tfm->base);

This happens to work because crypto_sync_skcipher is a trivial wrapper
around crypto_skcipher: &tfm->base is still 0 and crypto_free_skcipher()
handles that. Let's not rely on the layout of crypto_sync_skcipher.

This bug is found by a static analysis tool STCheck written by us.

Fixes: 69d6302b65a8 ("libceph: Remove VLA usage of skcipher").
Signed-off-by: Jia-Ju Bai <[e-mail]>
Reviewed-by: Ilya Dryomov <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

net/ceph/crypto.c

···
 	if (key) {
 		kfree(key->key);
 		key->key = NULL;
-		crypto_free_sync_skcipher(key->tfm);
-		key->tfm = NULL;
+		if (key->tfm) {
+			crypto_free_sync_skcipher(key->tfm);
+			key->tfm = NULL;
+		}
 	}
 }