evaluate: move interval flag compat check after set key evaluation

evaluate: move interval flag compat check after set key evaluation

Without this, included bogon asserts with:
BUG: unhandled key type 13
nft: src/intervals.c:73: setelem_expr_to_range: Assertion `0' failed.

... because we no longer evaluate set->key/data.

Move the check to the tail of the function, right before assiging
set->existing_set, so that set->key has been evaluated.

Fixes: ceab53cee499 ("evaluate: don't allow merging interval set/map with non-interval one")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>