http-walker: fix buffer underflow processing remote alternates

If we parse a remote alternates (or http-alternates), we
expect relative lines like:

  ../../foo.git/objects

which we convert into "$URL/../foo.git/" (and then use that
as a base for fetching more objects).

But if the remote feeds us nonsense like just:

  ../

we will try to blindly strip the last 7 characters, assuming
they contain the string "objects". Since we don't _have_ 7
characters at all, this results in feeding a small negative
value to strbuf_add(), which converts it to a size_t,
resulting in a big positive value. This should consistently
fail (since we can't generall allocate the max size_t minus
7 bytes), so there shouldn't be any security implications.

Let's fix this by using strbuf_strip_suffix() to drop the
characters we want. If they're not present, we'll ignore the
alternate (in theory we could use it as-is, but the rest of
the http-walker code unconditionally tacks "objects/" back
on, so it is it not prepared to handle such a case).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

http-walker.c

@@ -296,13 +296,16 @@ static void process_alternates_response(void *callback_data)
 					okay = 1;
 				}
 			}
-			/* skip "objects\n" at end */
 			if (okay) {
 				struct strbuf target = STRBUF_INIT;
 				strbuf_add(&target, base, serverlen);
-				strbuf_add(&target, data + i, posn - i - 7);
-
-				if (is_alternate_allowed(target.buf)) {
+				strbuf_add(&target, data + i, posn - i);
+				if (!strbuf_strip_suffix(&target, "objects")) {
+					warning("ignoring alternate that does"
+						" not end in 'objects': %s",
+						target.buf);
+					strbuf_release(&target);
+				} else if (is_alternate_allowed(target.buf)) {
 					warning("adding alternate object store: %s",
 						target.buf);
 					newalt = xmalloc(sizeof(*newalt));