ossl_param_build_set_multi_key_bn(): Do not set NULL BIGNUMs

This makes them zeroes otherwise
where NULLs actually mean the values aren't present.

Fixes #21935

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/22334)

crypto/param_build_set.c

@@ -101,21 +101,22 @@ int ossl_param_build_set_multi_key_bn(OSSL_PARAM_BLD *bld, OSSL_PARAM *params,
 {
     int i, sz = sk_BIGNUM_const_num(stk);
     OSSL_PARAM *p;
-
+    const BIGNUM *bn;
 
     if (bld != NULL) {
         for (i = 0; i < sz && names[i] != NULL; ++i) {
-            if (!OSSL_PARAM_BLD_push_BN(bld, names[i],
-                                        sk_BIGNUM_const_value(stk, i)))
+            bn = sk_BIGNUM_const_value(stk, i);
+            if (bn != NULL && !OSSL_PARAM_BLD_push_BN(bld, names[i], bn))
                 return 0;
         }
         return 1;
     }
 
     for (i = 0; i < sz && names[i] != NULL; ++i) {
+        bn = sk_BIGNUM_const_value(stk, i);
         p = OSSL_PARAM_locate(params, names[i]);
-        if (p != NULL) {
-            if (!OSSL_PARAM_set_BN(p, sk_BIGNUM_const_value(stk, i)))
+        if (p != NULL && bn != NULL) {
+            if (!OSSL_PARAM_set_BN(p, bn))
                 return 0;
         }
     }