BN_zero can't fail in LibreSSL >= 3.7.1.

Index: lib/libcmb/cmb.c
--- lib/libcmb/cmb.c.orig
+++ lib/libcmb/cmb.c
@@ -851,8 +851,7 @@ cmb_count_bn(struct cmb_config *config, uint32_t nitem
 	/* Initialize count */
 	if ((count = BN_new()) == NULL)
 		return (NULL);
-	if (!BN_zero(count))
-		goto cmb_count_bn_return;
+	BN_zero(count);
 
 	/* If entire set is requested, return 2^N[-1] */
 	if ((setinit == 1 && setdone == nitems) ||
@@ -1029,8 +1028,7 @@ cmb_bn(struct cmb_config *config, uint32_t nitems, cha
 	if (seq == NULL) {
 		if ((seq = BN_new()) == NULL)
 			goto cmb_bn_return;
-		if (!BN_zero(seq))
-			goto cmb_bn_return;
+		BN_zero(seq);
 	}
 
 	/* Show the empty set consisting of a single combination of no-items */