midx: check size of pack names chunk

We parse the pack-name chunk as a series of NUL-terminated strings. But
since we don't look at the chunk size, there's nothing to guarantee that
we don't parse off the end of the chunk (or even off the end of the
mapped file).

We can record the length, and then as we parse make sure that we never
walk past it.

The new test exercises the case, though note that it does not actually
segfault before this patch. It hits a NUL byte somewhere in one of the
other chunks, and comes up with a garbage pack name. You could construct
one that reads out-of-bounds (e.g., a PNAM chunk at the end of file),
but this case is simple and sufficient to check that we detect the
problem.

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

midx.h

@@ -32,6 +32,7 @@ struct multi_pack_index {
 	int local;
 
 	const unsigned char *chunk_pack_names;
+	size_t chunk_pack_names_len;
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_object_offsets;