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>
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index 19a2823..039509a 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -553,4 +553,14 @@
 		refs/tags/bogo refs/tags/master > actual &&
 	test_cmp expected actual
 '
+
+test_expect_success 'do not dereference NULL upon %(HEAD) on unborn branch' '
+	test_when_finished "git checkout master" &&
+	git for-each-ref --format="%(HEAD) %(refname:short)" refs/heads/ >actual &&
+	sed -e "s/^\* /  /" actual >expect &&
+	git checkout --orphan HEAD &&
+	git for-each-ref --format="%(HEAD) %(refname:short)" refs/heads/ >actual &&
+	test_cmp expect actual
+'
+
 test_done