pack-bitmap.c: fix broken warning() when missing MIDX'd pack

In commit 44f9fd6 (pack-bitmap.c: check preferred pack validity when
opening MIDX bitmap, 2022-05-24) we started opening all packs contained
within a MIDX when loading its corresponding bitmap.

However, if a pack is missing, then we will emit a warning like:

    warning: could not open pack pack-$HASH.pack

Later on commit f31a17c (pack-bitmap.c: open and store incremental
bitmap layers, 2025-03-20) updated this code to work with incremental
MIDX bitmaps, but did not adjust the index into the 'pack_names' field.

So if there is a pack in an incremental MIDX chain with a pack in a MIDX
layer with a non-zero number of packs in its base layer(s) (in other
words, any MIDX layer outside of the first one) that cannot be loaded,
we will do an out-of-bounds lookup.

Adjust the lookup into the 'pack_names' array by the number of packs in
the base to prevent a potential SIGSEGV here.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

pack-bitmap.c

@@ -490,7 +490,7 @@ static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
 	for (i = 0; i < bitmap_git->midx->num_packs + bitmap_git->midx->num_packs_in_base; i++) {
 		if (prepare_midx_pack(bitmap_repo(bitmap_git), bitmap_git->midx, i)) {
 			warning(_("could not open pack %s"),
-				bitmap_git->midx->pack_names[i]);
+				bitmap_git->midx->pack_names[i - bitmap_git->midx->num_packs_in_base]);
 			goto cleanup;
 		}
 	}