json: Work around segfault when encountering xt stmt

json: Work around segfault when encountering xt stmt

When trying to convert an xt stmt into JSON, print() callback was
called. Though the code in src/xt.c does not respect output_fp,
therefore buffer wasn't filled as expected making libjansson to puke:

| # nft -j list ruleset
| warning: stmt ops xt have no json callback
| nft: json.c:169: stmt_print_json: Assertion `__out' failed.
| Aborted (core dumped)

Avoid this by detecting xt stmt ops and returning a stub.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>