object: fix leak of shallow_stat

object: fix leak of shallow_stat

In eee4502baaf ("shallow: migrate shallow information into the object
parser", 2018-05-17), we added a stat_validity pointer into the
parsed_object_pool struct, but did not add code to free this in
parsed_object_pool_clear(). This leak was found by fuzz-commit-graph.

Clear the struct and then free it in parsed_object_pool_clear() to
prevent the leak.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>