openssl: Fix potential crash with ECDH on Windows

Apparently, we should use OPENSSL_free() to release memory allocated by
OpenSSL.  While it generally maps to free() that's apparently not the
case on Windows, where the ECP test vectors caused `ACCESS_VIOLATION
exception` crashes (not always the same vector).

Fixes: 74e02ff5e6 ("openssl: Mainly use EVP interface for ECDH")

src/libstrongswan/plugins/openssl/openssl_ec_diffie_hellman.c

@@ -246,7 +246,7 @@ METHOD(diffie_hellman_t, get_my_public_value, bool,
 	if (pub.len != 0)
 	{
 		*value = chunk_clone(chunk_skip(pub, 1));
-		chunk_free(&pub);
+		OPENSSL_free(pub.ptr);
 		return value->len != 0;
 	}
 	return FALSE;