parser_json: reject non-concat expression

parser_json: reject non-concat expression

Before "src: detach set, list and concatenation expression layout":
internal:0:0-0: Error: Concatenation with 0 elements is illegal

After this change, expr->size access triggers assert() failure, add
explicit test for etype to avoid this and error out:

internal:0:0-0: Error: Expected concat element, got symbol.

Fixes: e0d92243be1c ("src: detach set, list and concatenation expression layout")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>