Use bn_wexpand() rather than bn_expand() with sizeof(BN_ULONG).

This also fixes a bug in BN_MONT_CTX_set(), where the sizeof(BN_ULONG) in
the call to bn_expand() was not multiplied by eight (to get bits).

ok tb@

Author: jsing

src/lib/libcrypto/bn/bn_lib.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_lib.c,v 1.59 2022/11/23 03:04:52 jsing Exp $ */
+/* $OpenBSD: bn_lib.c,v 1.60 2022/11/23 03:10:10 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -518,7 +518,7 @@ int
 BN_set_word(BIGNUM *a, BN_ULONG w)
 {
 	bn_check_top(a);
-	if (bn_expand(a, (int)sizeof(BN_ULONG) * 8) == NULL)
+	if (bn_wexpand(a, 1) == NULL)
 		return (0);
 	a->neg = 0;
 	a->d[0] = w;

src/lib/libcrypto/bn/bn_mont.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mont.c,v 1.28 2022/02/07 19:44:23 tb Exp $ */
+/* $OpenBSD: bn_mont.c,v 1.29 2022/11/23 03:10:10 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -419,7 +419,7 @@ BN_MONT_CTX_set(BN_MONT_CTX *mont, const BIGNUM *mod, BN_CTX *ctx)
 		}
 		else /* if N mod word size == 1 */
 		{
-			if (bn_expand(Ri, (int)sizeof(BN_ULONG) * 2) == NULL)
+			if (bn_wexpand(Ri, 2) == NULL)
 				goto err;
 			/* Ri-- (mod double word size) */
 			Ri->neg = 0;