Fix minor oversights in nodeAgg.c.

aggstate->evalproj is always set up by ExecInitAgg, so there's no
need to test.  Doing so led Coverity to think that we might be
intending "slot" to be possibly NULL here, and it quite properly
complained that the rest of combine_aggregates() wasn't prepared
for that.

Also fix a couple of obvious thinkos in Asserts checking that
"inputoff" isn't past the end of the slot.

Errors introduced in commit 8ed3f11b, so no need for back-patch.