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>

pack-check.c

@@ -99,7 +99,8 @@ static int verify_packfile(struct repository *r,
 	for (i = 0; i < nr_objects; i++) {
 		entries[i].oid.hash = nth_packed_object_sha1(p, i);
 		if (!entries[i].oid.hash)
-			die("internal error pack-check nth-packed-object");
+			BUG("unable to get oid of object %lu from %s",
+			    (unsigned long)i, p->pack_name);
 		entries[i].offset = nth_packed_object_offset(p, i);
 		entries[i].nr = i;
 	}