midx-write.c: tolerate `--preferred-pack` without bitmaps

When passing a preferred pack to the MIDX write machinery, we ensure that the given preferred pack is non-empty since 5d3cd09a808 (midx: reject empty `--preferred-pack`'s, 2021-08-31). However packs are only loaded (via `write_midx_internal()`, though a subsequent patch will refactor this code out to its own function) when the `MIDX_WRITE_REV_INDEX` flag is set. So if a caller runs: $ git multi-pack-index write --preferred-pack=... with both (a) an existing MIDX, and (b) specifies a pack from that MIDX as the preferred one, without passing `--bitmap`, then the check added in 5d3cd09a808 will result in a segfault. Note that packs loaded from disk which don't appear in an existing MIDX do not trigger this issue, as those packs are loaded unconditionally. We conditionally load packs from a MIDX since we tolerate MIDXs whose packs do not resolve (i.e., via the MIDX write after removing unreferenced packs via 'git multi-pack-index expire'). In practice, this isn't possible to trigger when running `git multi-pack-index write` from `git repack`, as the latter always passes `--stdin-packs`, which prevents us from loading an existing MIDX, as it forces all packs to be read from disk. But a future commit in this series will change that behavior to unconditionally load an existing MIDX, even with `--stdin-packs`, making this behavior trigger-able from 'repack' much more easily. Prevent this from being an issue by removing the segfault altogether by calling `prepare_midx_pack()` on packs loaded from an existing MIDX when either the `MIDX_WRITE_REV_INDEX` flag is set *or* we specified a `--preferred-pack`. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>