remote-curl: Add a format check to parsing of info/refs

When parsing info/refs, no checks were applied that the file was in
the requried format.  Since the file is read from a remote webserver,
this isn't guarenteed to be true.  Add a check that the file at least
only contains lines that consist of 40 characters followed by a tab
and then the ref name.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote-curl.c

@@ -227,6 +227,8 @@ static struct ref *parse_info_refs(struct discovery *heads)
 		if (data[i] == '\t')
 			mid = &data[i];
 		if (data[i] == '\n') {
+			if (mid - start != 40)
+				die("%sinfo/refs not valid: is this a git repository?", url);
 			data[i] = 0;
 			ref_name = mid + 1;
 			ref = xmalloc(sizeof(struct ref) +