The chunk-wise aggregation pushdown code creates a copy of the existing
paths; it copies the existing paths to create new ones with pushed-down
aggregates. However, the copy_merge_append_path function behaves
differently than other copy functions (e.g., copy_append_path); it
resets the pathtarget on the copy. This leads to a wrong pathlist and
crashes. This PR fixes the wrong pathtarget by setting it after the path
is copied.
