selftests: netfilter: nft_concat_range.sh: send packets to empty set

The selftest doesn't cover this error path:
 scratch = *raw_cpu_ptr(m->scratch);
 if (unlikely(!scratch)) { // here

cover this too.

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

Author: Florian Westphal

tools/testing/selftests/net/netfilter/nft_concat_range.sh

@@ -1311,6 +1311,9 @@ maybe_send_match() {
 # - remove some elements, check that packets don't match anymore
 test_correctness_main() {
 	range_size=1
+
+	send_nomatch $((end + 1)) $((end + 1 + src_delta)) || return 1
+
 	for i in $(seq "${start}" $((start + count))); do
 		local elem=""