parser_json: release buffer returned by json_dumps

parser_json: release buffer returned by json_dumps

The signature of `json_dumps` is:

`char *json_dumps(const json_t *json, size_t flags)`:

It will return a pointer to an owned string, the caller must free it.
However, `json_error` just borrows the string to format it as `%s`, but
after printing the formatted error message, the pointer to the string is
lost and thus never freed.

Fixes: 586ad210368b ("libnftables: Implement JSON parser")
Signed-off-by: Sebastian Walz (sivizius) <sebastian.walz@secunet.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>