dh: allow a zero-size export buffer - #477

Problem

wp_dh_export() computes a single allocation covering both the group data and the key pair, then requires OPENSSL_secure_malloc() to return non-NULL before it exports anything. For a parameter-only named-group key, both contributions are zero: wp_dh_export_group_alloc_size() returns 0 because a named group is exported as a string pointer with no backing buffer, and wp_dh_export_keypair_alloc_size() returns 0 when there are no public or private bytes. OPENSSL_secure_malloc(0) returns NULL, so a key that needed no buffer at all fails to export.

Reachable through EVP_PKEY_todata() and through any cross-provider export of DH domain parameters. Closes f-11557.

Fix (src/wp_dh_kmgmt.c)

Allocate only when the computed size is non-zero:

 /* Named group parameters need no buffer - zero size is valid. */
 if (sz > 0) {
 data = OPENSSL_secure_malloc(sz);
 if (data == NULL) {
 ok = 0;
 }
 }

data stays NULL on that path and is never dereferenced: wp_dh_export_group() touches it only in its explicit-parameters branch, wp_dh_export_keypair() is guarded by pubSz > 0 / privSz > 0, and OPENSSL_secure_clear_free() returns early on a null pointer.

Tests

test_dh_export_named_group_params imports an ffdhe2048 parameter-only key via EVP_PKEY_fromdata(), exports it with EVP_PKEY_todata(), and requires a group name in the result — so it fails on a silently empty export as well as on the error return.

Verification

• Build clean under -Werror; no new compiler warnings.
• Full unit suite: 200/200 pass. DH cases: 15/15.
• Negative control: with the guard reverted the new test fails at Export of parameter-only named group key failed; it passes with the guard in place.