Check for zero modulus in BN_MONT_CTX_set().

From OpenSSL 6a009812, prompted by a report by Guido Vranken

ok beck jsing

Author: tb

src/lib/libcrypto/bn/bn_mont.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mont.c,v 1.27 2021/12/04 16:05:46 tb Exp $ */
+/* $OpenBSD: bn_mont.c,v 1.28 2022/02/07 19:44:23 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -369,6 +369,9 @@ BN_MONT_CTX_set(BN_MONT_CTX *mont, const BIGNUM *mod, BN_CTX *ctx)
 	int ret = 0;
 	BIGNUM *Ri, *R;
 
+	if (BN_is_zero(mod))
+		return 0;
+
 	BN_CTX_start(ctx);
 	if ((Ri = BN_CTX_get(ctx)) == NULL)
 		goto err;