remote: handle broken symrefs

It's possible for resolve_ref_unsafe() to return NULL with a REF_ISSYMREF flag if a symref points to a broken ref. In this case, the read_remote_branches() function will segfault passing the name to xstrdup(). This is hard to trigger in practice, since this function is used as a callback to for_each_ref(), which will skip broken refs in the first place (so it would have to be broken racily, or for us to see a transient filesystem error). If we see such a racy broken outcome let's treat it as "not a symref". This is exactly the same thing that would happen in the non-racy case (our function would not be called at all, as for_each_ref would skip the broken symref). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>