JSON: Review set elem expressions

JSON: Review set elem expressions

* There is no need to prefix element-specific properties with 'elem_',
  they can't conflict.
* In json_parse_set_stmt(), searching for above properties is pointless
  since that's already done by called function.
* Fix potential NULL-pointer deref in json_parse_set_elem_expr_stmt():
  json_parse_flagged_expr() may return NULL.

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