unix: avoid nil pointer dereference in Utime

Passing a nil buf *Utimbuf to Utime on linux/{arm,arm64,loong64,riscv64}
will cause a nil pointer dereference. Check buf explicitly to avoid this
case.

Change-Id: Ifb719f83029002f043919daa066af6b1fb4ebe03
Reviewed-on: https://go-review.googlesource.com/c/sys/+/764562
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

unix/syscall_linux_arm.go

@@ -82,6 +82,9 @@ func Time(t *Time_t) (Time_t, error) {
 }
 
 func Utime(path string, buf *Utimbuf) error {
+	if buf == nil {
+		return Utimes(path, nil)
+	}
 	tv := []Timeval{
 		{Sec: buf.Actime},
 		{Sec: buf.Modtime},

unix/syscall_linux_arm64.go

@@ -113,6 +113,9 @@ func Time(t *Time_t) (Time_t, error) {
 }
 
 func Utime(path string, buf *Utimbuf) error {
+	if buf == nil {
+		return Utimes(path, nil)
+	}
 	tv := []Timeval{
 		{Sec: buf.Actime},
 		{Sec: buf.Modtime},

unix/syscall_linux_loong64.go

@@ -150,6 +150,9 @@ func Time(t *Time_t) (Time_t, error) {
 }
 
 func Utime(path string, buf *Utimbuf) error {
+	if buf == nil {
+		return Utimes(path, nil)
+	}
 	tv := []Timeval{
 		{Sec: buf.Actime},
 		{Sec: buf.Modtime},

unix/syscall_linux_riscv64.go

@@ -112,6 +112,9 @@ func Time(t *Time_t) (Time_t, error) {
 }
 
 func Utime(path string, buf *Utimbuf) error {
+	if buf == nil {
+		return Utimes(path, nil)
+	}
 	tv := []Timeval{
 		{Sec: buf.Actime},
 		{Sec: buf.Modtime},