crypto: api - Fix crypto_drop_spawn crash on blank spawns

This patch allows crypto_drop_spawn to be called on spawns that
have not been initialised or have failed initialisation.  This
fixes potential crashes during initialisation without adding
special case code.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/algapi.c

@@ -507,6 +507,9 @@ EXPORT_SYMBOL_GPL(crypto_init_spawn2);
 
 void crypto_drop_spawn(struct crypto_spawn *spawn)
 {
+	if (!spawn->alg)
+		return;
+
 	down_write(&crypto_alg_sem);
 	list_del(&spawn->list);
 	up_write(&crypto_alg_sem);