monitor: do not call interval_map_decompose() for concat intervals

monitor: do not call interval_map_decompose() for concat intervals

Without this, nft monitor will either print garbage or even segfault
when encountering a concat set because we pass expr->value to libgmp
helpers for concat (non-value) expressions.

Also, for concat case, we need to call concat_range_aggregate() helper.
Add a test case for this.  Without this patch, it gives:

tests/monitor/run-tests.sh: line 98: 1163 Segmentation fault
(core dumped) $nft -nn -e -f $command_file > $echo_output

Signed-off-by: Florian Westphal <fw@strlen.de>