[libcmb] GCC error for use of void BN_zero() with OpenSSL

[zmughal]

I get the following error with gcc 12.

gcc -fPIC -Wall -I. -I/usr/local/include -DHAVE_CONFIG_H -c cmb.c -o cmb.o
cmb.c: In function ‘cmb_count_bn’:
cmb.c:854:13: error: invalid use of void expression
  854 |         if (!BN_zero(count))
      |             ^
cmb.c: In function ‘cmb_bn’:
cmb.c:1032:21: error: invalid use of void expression
 1032 |                 if (!BN_zero(seq))
      |                     ^
make: *** [GNUmakefile:39: cmb.o] Error 1

---

Per the OpenSSL docs:

void BN_zero(BIGNUM *a);

...
In OpenSSL 0.9.8, BN_zero() was changed to not return a value; previous versions returned an int.

This can be fixed by enabling the deprecated API using OPENSSL_API_COMPAT, but this is incompatible with LibreSSL where the deprecated API is still available under the name BN_zero (source):

int
BN_zero(BIGNUM *a);

---

One approach is to switch to the new API BN_zero_ex() which appeared in OpenSSL 0.9.8. Another is to use BN_set_word as here:

#  define BN_zero(a)      (BN_set_word((a),0))

which is compatible with older versions of OpenSSL.