object: fix leak of shallow_stat

In eee4502b ("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: NJosh Steadmon <steadmon@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

object.c

@@ -557,9 +557,11 @@ void parsed_object_pool_clear(struct parsed_object_pool *o)
 	clear_alloc_state(o->commit_state);
 	clear_alloc_state(o->tag_state);
 	clear_alloc_state(o->object_state);
+	stat_validity_clear(o->shallow_stat);
 	FREE_AND_NULL(o->blob_state);
 	FREE_AND_NULL(o->tree_state);
 	FREE_AND_NULL(o->commit_state);
 	FREE_AND_NULL(o->tag_state);
 	FREE_AND_NULL(o->object_state);
+	FREE_AND_NULL(o->shallow_stat);
 }