avoid a false-positive assert from rebinding module

Somehow I didn't notice this field when adding ::add_selected.
We probably never put anything into answer's ADDITIONAL,
so noone's noticed a problem until now.

We currently allow modules to produce a packet beforehand,
but the possibility was rather implicit in this part of code.
In particular, we might possible trigger a false alarm in
assert(answer->current <= KNOT_ANSWER)