netlink_delinearize: meta l4proto range printing broken on 32bit

netlink_delinearize: meta l4proto range printing broken on 32bit

Florian Westphal says:

09565a4b1ed4863d44c4509a93c50f44efd12771 ("netlink_delinearize: consolidate
range printing") causes nft to segfault on 32bit machine when printing l4proto
ranges.

The problem is that meta_expr_pctx_update() assumes that right is a value, but
after this change it can also be a range.

Thus, expr->value contents are undefined (its union).  On x86_64 this is also
broken but by virtue of struct layout and pointer sizes, value->_mp_size will
almost always be 0 so mpz_get_uint8() returns 0.

But on x86-32 _mp_size will be huge value (contains expr->right pointer of
range), so we crash in libgmp.

Pablo says:

We shouldn't call pctx_update(), before the transformation we had
there a expr->op == { OP_GT, OP_GTE, OP_LT, OP_LTE }. So we never
entered that path as the assert in payload_expr_pctx_update()
indicates.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Florian Westphal <fw@strlen.de>