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>