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: NJunio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1017,7 +1017,7 @@ static void populate_value(struct ref_array_item *ref)
 
 			head = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
 						  sha1, NULL);
-			if (!strcmp(ref->refname, head))
+			if (head && !strcmp(ref->refname, head))
 				v->s = "*";
 			else
 				v->s = " ";

t/t6300-for-each-ref.sh

@@ -553,4 +553,14 @@ test_expect_success 'Verify sort with multiple keys' '
 		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