Fix feature id bookkeeping in count handler

[paleolimbot]

Fixes #139.

wk::wk_count(wk::wkt())
#> [1] n_geom  n_ring  n_coord
#> <0 rows> (or 0-length row.names)

^{Created on 2022-09-19 by the reprex package (v2.0.1)}