builtin/clone: fix segfault when using --revision with protocol v0

builtin/clone: fix segfault when using --revision with protocol v0

Servers supporting protocol v2 do not advertise excess refs and honor
`transport_ls_refs_options.ref_prefixes` when

    $ git clone --revision=refs/heads/main $URL

contacts them, but when talking to a server that does not support
protocol v2 the client segfaults. This can also be observed when v0 is
enforced for example by

    $ git -c protocol.version=0 clone --revision=refs/heads/main $URL

In the protocol v2 case the server honors
`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 protocol v0 case 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.

Signed-off-by: Adrian Friedli <adrian.friedli@mt.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>