pack-check: convert "internal error" die to a BUG()
If we fail to load the oid from the index of a packfile, we'll die()
with an "internal error". But this should never happen: we'd fail here
only if the idx needed to be lazily opened (but we've already opened it)
or if we asked for an out-of-range index (but we're iterating using the
same count that we'd check the range against). A corrupted index might
have a bogus count (e.g., too large for its size), but we'd have
complained and aborted already when opening the index initially.
While we're here, we can add a few details so that if this bug ever
_does_ trigger, we'll have a bit more information.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>