crypto: asymmetric_keys - fix uninitialized pointers with free attribute

Uninitialized pointers with `__free` attribute can cause undefined
behavior as the memory assigned randomly to the pointer is freed
automatically when the pointer goes out of scope.

crypto/asymmetric_keys doesn't have any bugs related to this as of now,
but, it is better to initialize and assign pointers with `__free`
attribute in one statement to ensure proper scope-based cleanup

Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Ally Heev <[e-mail]>
Reviewed-by: Ignat Korchagin <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/asymmetric_keys/x509_cert_parser.c

···
  */
 struct x509_certificate *x509_cert_parse(const void *data, size_t datalen)
 {
-	struct x509_certificate *cert __free(x509_free_certificate);
+	struct x509_certificate *cert __free(x509_free_certificate) = NULL;
 	struct x509_parse_context *ctx __free(kfree) = NULL;
 	struct asymmetric_key_id *kid;
 	long ret;

crypto/asymmetric_keys/x509_public_key.c

···
  */
 static int x509_key_preparse(struct key_preparsed_payload *prep)
 {
-	struct x509_certificate *cert __free(x509_free_certificate);
+	struct x509_certificate *cert __free(x509_free_certificate) = NULL;
 	struct asymmetric_key_ids *kids __free(kfree) = NULL;
 	char *p, *desc __free(kfree) = NULL;
 	const char *q;