pack-objects: use BUG(...) not die("BUG: ...")

pack-objects: use BUG(...) not die("BUG: ...")

Change this code added in da93d12b004 (pack-objects: be incredibly
anal about stdio semantics, 2006-04-02) to use BUG() instead.

See 1a07e59c3e2 (Update messages in preparation for i18n, 2018-07-21)
for when the "BUG: " prefix was added, and [1] for background on the
Solaris behavior that prompted the exhaustive error checking in this
fgets() loop.

1. https://lore.kernel.org/git/824.1144007555@lotus.CS.Berkeley.EDU/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>