various: add missing clear_pathspec(), fix leaks

Fix memory leaks resulting from a missing clear_pathspec().

- archive.c: Plug a leak in the "struct archiver_args", and
  clear_pathspec() the "pathspec" member that the "parse_pathspec_arg()"
  call in this function populates.

- builtin/clean.c: Fix a memory leak that's been with us since
  893d839970c (clean: convert to use parse_pathspec, 2013-07-14).

- builtin/reset.c: Add clear_pathspec() calls to cmd_reset(),
  including to the codepaths where we'd return early.

- builtin/stash.c: Call clear_pathspec() on the pathspec initialized
  in push_stash().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
