Add a basic test for BN_bn2dec.

This would have caught 099e2968. This is
a port of the test added in
https://boringssl.googlesource.com/boringssl/+/7c040756178e14a4d181b6d93abb3827c93189c4Reviewed-by: NEmilia Käsper <emilia@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1496)

test/bntest.c

@@ -83,6 +83,7 @@ int test_gf2m_mod_solve_quad(BIO *bp, BN_CTX *ctx);
 int test_kron(BIO *bp, BN_CTX *ctx);
 int test_sqrt(BIO *bp, BN_CTX *ctx);
 int test_small_prime(BIO *bp, BN_CTX *ctx);
+int test_bn2dec(BIO *bp);
 int rand_neg(void);
 static int results = 0;
 
@@ -260,6 +261,11 @@ int main(int argc, char *argv[])
         goto err;
     (void)BIO_flush(out);
 
+    message(out, "BN_bn2dec");
+    if (!test_bn2dec(out))
+        goto err;
+    (void)BIO_flush(out);
+
 #ifndef OPENSSL_NO_EC2M
     message(out, "BN_GF2m_add");
     if (!test_gf2m_add(out))
@@ -1839,6 +1845,52 @@ int test_small_prime(BIO *bp, BN_CTX *ctx)
     return ret;
 }
 
+int test_bn2dec(BIO *bp)
+{
+    static const char *bn2dec_tests[] = {
+        "0",
+        "1",
+        "-1",
+        "100",
+        "-100",
+        "123456789012345678901234567890",
+        "-123456789012345678901234567890",
+        "123456789012345678901234567890123456789012345678901234567890",
+        "-123456789012345678901234567890123456789012345678901234567890",
+    };
+    int ret = 0;
+    size_t i;
+    BIGNUM *bn = NULL;
+    char *dec = NULL;
+
+    for (i = 0; i < OSSL_NELEM(bn2dec_tests); i++) {
+        if (!BN_dec2bn(&bn, bn2dec_tests[i]))
+            goto err;
+
+        dec = BN_bn2dec(bn);
+        if (dec == NULL) {
+            fprintf(stderr, "BN_bn2dec failed on %s.\n", bn2dec_tests[i]);
+            goto err;
+        }
+
+        if (strcmp(dec, bn2dec_tests[i]) != 0) {
+            fprintf(stderr, "BN_bn2dec gave %s, wanted %s.\n", dec,
+                    bn2dec_tests[i]);
+            goto err;
+        }
+
+        OPENSSL_free(dec);
+        dec = NULL;
+    }
+
+    ret = 1;
+
+err:
+    BN_free(bn);
+    OPENSSL_free(dec);
+    return ret;
+}
+
 int test_lshift(BIO *bp, BN_CTX *ctx, BIGNUM *a_)
 {
     BIGNUM *a, *b, *c, *d;