log: handle broken HEAD in decoration check

The resolve_ref_unsafe() function may return NULL even with a REF_ISSYMREF flag if a symref points to a broken ref. As a result, it's possible for the decoration code's "is this branch the current HEAD" check to segfault when it passes the NULL to starts_with(). This is unlikely in practice, since we can only reach this code if we already resolved HEAD to a matching sha1 earlier. But it's possible if HEAD racily becomes broken, or if there's a transient filesystem error. We can fix this by returning early in the broken case, since NULL could not possibly match any of our branch names. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>