midx: check size of object offset chunk

The object offset chunk has one fixed-size entry for each object in the
midx. But since we don't check its size, we may access out-of-bounds
memory if we see a corrupt or malicious midx file.

Sine the entries are fixed-size, the total length can be known up-front,
and we can just check it while parsing the chunk (this is similar to
what we do when opening pack idx files, which contain a similar offset
table).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5319-multi-pack-index.sh

@@ -1108,4 +1108,14 @@ test_expect_success 'reader handles unaligned chunks' '
 	test_cmp expect.err err
 '
 
+test_expect_success 'reader notices too-small object offset chunk' '
+	corrupt_chunk OOFF clear 00000000 &&
+	test_must_fail git log 2>err &&
+	cat >expect <<-\EOF &&
+	error: multi-pack-index object offset chunk is the wrong size
+	fatal: multi-pack-index required object offsets chunk missing or corrupted
+	EOF
+	test_cmp expect err
+'
+
 test_done