segtree: incorrect type when aggregating concatenated set ranges

segtree: incorrect type when aggregating concatenated set ranges

Uncovered by the compound_expr_remove() replacement by type safe function
coming after this patch.

Add expression to the concatenation which is reachable via expr_value().

This bug is subtle, I could not spot any reproducible buggy behaviour
when using the wrong type when running the existing tests.

Fixes: 8ac2f3b2fca3 ("src: Add support for concatenated set ranges")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>