openssl: Don't allocate salt if PRF/hash is unknown

This can happen if e.g. AES-XCBC is selected.

Fixes: 2dbeecfc02 ("openssl: Fix testing KDF_PRF in the constructor with OpenSSL 3.5.1")

src/libstrongswan/plugins/openssl/openssl_kdf.c

@@ -203,7 +203,7 @@ kdf_t *openssl_kdf_create(key_derivation_function_t algo, va_list args)
 
 	/* also generate a salt (as if none was provided, i.e. zeroes of hash length)
 	 * as OpenSSL 3.5.1+ won't accept NULL anymore */
-	if (algo == KDF_PRF)
+	if (algo == KDF_PRF && this->hasher)
 	{
 		this->salt = chunk_copy_pad(chunk_alloc(get_length(this)),
 									chunk_empty, 0);