json: Fix for memleak in __binop_expr_json

json: Fix for memleak in __binop_expr_json

When merging the JSON arrays generated for LHS and RHS of nested binop
expressions, the emptied array objects leak if their reference is not
decremented.

Fix this and tidy up other spots which did it right already by
introducing a json_array_extend wrapper.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Fixes: 0ac39384fd9e4 ("json: Accept more than two operands in binary expressions")
Signed-off-by: Phil Sutter <phil@nwl.cc>