parser_json: Fix error reporting for invalid syntax

parser_json: Fix error reporting for invalid syntax

Errors emitted by the JSON parser caused BUG() in erec_print() due to
input descriptor values being bogus.

Due to lack of 'include' support, JSON parser uses a single input
descriptor only and it lived inside the json_ctx object on stack of
nft_parse_json_*() functions.

By the time errors are printed though, that scope is not valid anymore.
Move the static input descriptor object to avoid this.

Fixes: 586ad210368b7 ("libnftables: Implement JSON parser")
Signed-off-by: Phil Sutter <phil@nwl.cc>