add-interactive: plug a leak in get_untracked_files

Plug a leak we have since ab1e1cccaf (built-in add -i: re-implement
`add-untracked` in C, 2019-11-29).

This leak can be triggered with:

	$ echo a | git add -i

As a curiosity, we have a somewhat similar function in builtin/stash.c,
which correctly frees the memory.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.c

@@ -865,6 +865,7 @@ static int get_untracked_files(struct repository *r,
 	}
 
 	strbuf_release(&buf);
+	dir_clear(&dir);
 	return 0;
 }