Merge branch 'tb/midx-race-in-pack-objects'

Merge branch 'tb/midx-race-in-pack-objects'

The multi-pack-index code did not protect the packfile it is going
to depend on from getting removed while in use, which has been
corrected.

* tb/midx-race-in-pack-objects:
  builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects
  builtin/pack-objects.c: ensure included `--stdin-packs` exist
  builtin/pack-objects.c: avoid redundant NULL check
  pack-bitmap.c: check preferred pack validity when opening MIDX bitmap