fetch-pack: fix out-of-bounds buffer offset in get_ack

When we read acks from the remote, we expect either:

  ACK <sha1>

or

  ACK <sha1> <multi-ack-flag>

We parse the "ACK <sha1>" bit from the line, and then start
looking for the flag strings at "line+45"; if we don't have
them, we assume it's of the first type.  But if we do have
the first type, then line+45 is not necessarily inside our
string at all!

It turns out that this works most of the time due to the way
we parse the packets. They should come in with a newline,
and packet_read puts an extra NUL into the buffer, so we end
up with:

  ACK <sha1>\n\0

with the newline at offset 44 and the NUL at offset 45. We
then strip the newline, putting a NUL at offset 44. So
when we look at "line+45", we are looking past the end of
our string; but it's OK, because we hit the terminator from
the original string.

This breaks down, however, if the other side does not
terminate their packets with a newline. In that case, our
packet is one character shorter, and we start looking
through uninitialized memory for the flag. No known
implementation sends such a packet, so it has never come up
in practice.

This patch tightens the check by looking for a short,
flagless ACK before trying to parse the flag.

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

fetch-pack.c

@@ -226,6 +226,8 @@ static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 		return NAK;
 	if (!prefixcmp(line, "ACK ")) {
 		if (!get_sha1_hex(line+4, result_sha1)) {
+			if (len < 45)
+				return ACK;
 			if (strstr(line+45, "continue"))
 				return ACK_continue;
 			if (strstr(line+45, "common"))