builtin/pack-objects.c: ensure included `--stdin-packs` exist

A subsequent patch will teach `want_object_in_pack()` to set its
`*found_pack` and `*found_offset` poitners to NULL when the provided
pack does not pass the `is_pack_valid()` check.

The `--stdin-packs` mode of `pack-objects` is not quite prepared to
handle this. To prepare it for this change, do the following two things:

  - Ensure provided packs pass the `is_pack_valid()` check when
    collecting the caller-provided packs into the "included" and
    "excluded" lists.

  - Gracefully handle any _invalid_ packs being passed to
    `want_object_in_pack()`.

Calling `is_pack_valid()` early on makes it substantially less likely
that we will have to deal with a pack going away, since we'll have an
open file descriptor on its contents much earlier.

But even packs with open descriptors can become invalid in the future if
we (a) hit our open descriptor limit, forcing us to close some open
packs, and (b) one of those just-closed packs has gone away in the
meantime.

`add_object_entry_from_pack()` depends on having a non-NULL
`*found_pack`, since it passes that pointer to `packed_object_info()`,
meaning that we would SEGV if the pointer became NULL (like we propose
to do in `want_object_in_pack()` in the following patch).

But avoiding calling `packed_object_info()` entirely is OK, too, since
its only purpose is to identify which objects in the included packs are
commits, so that they can form the tips of the advisory traversal used
to discover the object namehashes.

Failing to do this means that at worst we will produce lower-quality
deltas, but it does not prevent us from generating the pack as long as
we can find a copy of each object from the disappearing pack in some
other part of the repository.

Co-authored-by: Victoria Dye <vdye@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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