branch_get_push: do not segfault when HEAD is detached

Move the detached HEAD check from branch_get_push_1() to
branch_get_push() to avoid setting branch->push_tracking_ref when
branch is NULL.
Signed-off-by: NKyle Meyer <kyle@kyleam.com>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1717,9 +1717,6 @@ static const char *branch_get_push_1(struct branch *branch, struct strbuf *err)
 {
 	struct remote *remote;
 
-	if (!branch)
-		return error_buf(err, _("HEAD does not point to a branch"));
-
 	remote = remote_get(pushremote_for_branch(branch, NULL));
 	if (!remote)
 		return error_buf(err,
@@ -1779,6 +1776,9 @@ static const char *branch_get_push_1(struct branch *branch, struct strbuf *err)
 
 const char *branch_get_push(struct branch *branch, struct strbuf *err)
 {
+	if (!branch)
+		return error_buf(err, _("HEAD does not point to a branch"));
+
 	if (!branch->push_tracking_ref)
 		branch->push_tracking_ref = branch_get_push_1(branch, err);
 	return branch->push_tracking_ref;

t/t1514-rev-parse-push.sh

@@ -60,4 +60,10 @@ test_expect_success '@{push} with push refspecs' '
 	resolve topic@{push} refs/remotes/origin/magic/topic
 '
 
+test_expect_success 'resolving @{push} fails with a detached HEAD' '
+	git checkout HEAD^0 &&
+	test_when_finished "git checkout -" &&
+	test_must_fail git rev-parse @{push}
+'
+
 test_done