Fix some ancient silliness with a random byte

For nearly 25 years this test has attempted to generate random numbers of
bit length between 192 and 319 bits. Unfortunately, it used an unsigned
char so instead of generating numbers in the interval [-64..63] and add
them to 256, it used numbers in the intervals [0..63] and [192..255]...

Author: tb

src/regress/lib/libcrypto/bn/bn_mod_exp.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: bn_mod_exp.c,v 1.5 2022/12/02 18:31:40 tb Exp $	*/
+/*	$OpenBSD: bn_mod_exp.c,v 1.6 2022/12/03 08:05:52 tb Exp $	*/
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -73,7 +73,7 @@ main(int argc, char *argv[])
 	BIGNUM *r_mont, *r_mont_const, *r_recp, *r_simple;
 	BIGNUM *r_mont_ct, *r_mont_nonct, *a, *b, *m;
 	BN_CTX *ctx;
-	unsigned char c;
+	int c;
 	int i, ret;
 
 	ERR_load_BN_strings();
@@ -103,18 +103,15 @@ main(int argc, char *argv[])
 		goto err;
 
 	for (i = 0; i < 200; i++) {
-		arc4random_buf(&c, 1);
-		c = (c % BN_BITS) - BN_BITS2;
+		c = (arc4random() % BN_BITS) - BN_BITS2;
 		if (!BN_rand(a, NUM_BITS + c, 0, 0))
 			goto err;
 
-		arc4random_buf(&c, 1);
-		c = (c % BN_BITS) - BN_BITS2;
+		c = (arc4random() % BN_BITS) - BN_BITS2;
 		if (!BN_rand(b, NUM_BITS + c, 0, 0))
 			goto err;
 
-		arc4random_buf(&c, 1);
-		c = (c % BN_BITS) - BN_BITS2;
+		c = (arc4random() % BN_BITS) - BN_BITS2;
 		if (!BN_rand(m, NUM_BITS + c, 0, 1))
 			goto err;