evaluate: fix assertion failure with malformed map definitions

evaluate: fix assertion failure with malformed map definitions

Included bogon triggers:
nft: src/evaluate.c:2267: expr_evaluate_mapping: Assertion `set->data != NULL' failed.

After this fix, following errors will be shown:
Error: unqualified type invalid specified in map definition. Try "typeof expression" instead of "type datatype".
map m {
    ^
map m {
    ^
Error: map has no mapping data

Fixes: 343a51702656 ("src: store expr, not dtype to track data in sets")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>