[PATCH] builtin/clone: fix segfault when using --revision on some servers

[Adrian Friedli <adrian.friedli@mt.com>]

Fix a segfault when a server advertises more refs than requested when
using the --revision argument.

In the good case the server respects
`transport_ls_refs_options.ref_prefixes` and in `cmd_clone()` the linked
list `refs` returned by `transport_get_remote_refs()` only contains a
single item, which is the ref requested with the --revision argument.
Both `remote_head` returned by `find_ref_by_name()` and
`remote_head_points_at` returned by `guess_remote_head()` are NULL. The
guard in `update_remote_refs()` skips a the affected code because
`remote_head_points_at` is NULL.

In the bad case the server ignores
`transport_ls_refs_options.ref_prefixes` and in `cmd_clone()` the linked
list `refs` returned by `transport_get_remote_refs()` contains many
items, amongst others "HEAD". `remote_head` returned by
`find_ref_by_name()` is not NULL and `remote_head_points_at` returned by
`guess_remote_head()` is not NULL but its field `peer_ref` is NULL.
Because `remote_head_points_at` is not NULL the guard in
`update_remote_refs()` does not skip the affected code and
`remote_head_points_at->peer_ref->name` is accessed, which causes a
segfault later on.

Extend the guard in `update_remote_refs()` to also skip the block of
code if `remote_head_points_at->peer_ref` is NULL.

Signed-off-by: Adrian Friedli <adrian.friedli@mt.com>
---
The segfault can be reproduced by e.g.

git clone --revision=refs/heads/main \
https://dev.azure.com/public-git/sample/_git/sample

 builtin/clone.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/builtin/clone.c b/builtin/clone.c
index fba3c9c508..09219791da 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -557,7 +557,7 @@ static void update_remote_refs(const struct ref *refs,
 			write_followtags(refs, msg);
 	}
 
-	if (remote_head_points_at && !option_bare) {
+	if (remote_head_points_at && remote_head_points_at->peer_ref && !option_bare) {
 		struct strbuf head_ref = STRBUF_INIT;
 		strbuf_addstr(&head_ref, branch_top);
 		strbuf_addstr(&head_ref, "HEAD");
-- 
2.53.0.394.g500c12b044