midx-write.c: assume checksum-invalid MIDXs require an update

In 6ce9d558ced (midx-write: skip rewriting MIDX with `--stdin-packs`
unless needed, 2025-12-10), the MIDX machinery learned how to optimize
out unnecessary writes with "--stdin-packs".

In order to do this, it compares the contents of the in-progress write
against a MIDX loaded directly from the object store. We load a separate
MIDX (as opposed to checking our update relative to "ctx.m") because the
MIDX code does not reuse an existing MIDX with --stdin-packs, and always
leaves "ctx.m" as NULL. See commit 0c5a62f14bc (midx-write.c: do not
read existing MIDX with `packs_to_include`, 2024-06-11) for details on
why.

If "ctx.m" is non-NULL, however, it is guaranteed to be checksum-valid,
since we only assign "ctx.m" when "midx_checksum_valid()" returns true.
Since the same guard does not exist for the MIDX we pass to
"midx_needs_update()", we may ignore on-disk corruption when determining
whether or not we can optimize out the write.

Add a similar guard within "midx_needs_update()" to prevent such an
issue.

A more robust fix would involve revising 0c5a62f14bc and teaching the
MIDX generation code how to reuse an existing MIDX even when invoked
with "--stdin-packs", such that we could avoid side-loading the MIDX
directly from the object store in order to call "midx_needs_update()".
For now, pursue the minimal fix.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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