for-each-ref: do not segv with %(HEAD) on an unborn branch

for-each-ref: do not segv with %(HEAD) on an unborn branch

The code to flip between "*" and " " prefixes depending on what
branch is checked out used in --format='%(HEAD)' did not consider
that HEAD may resolve to an unborn branch and dereferenced a NULL.

This will become a lot easier to trigger as the codepath will be
used to reimplement "git branch [--list]" in the future.

Signed-off-by: Junio C Hamano <gitster@pobox.com>