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>