syscall: fix nil pointer dereference in Select on linux/{arm64,mips64x}

The timeout parameter might be nil, don't dereference it
unconditionally.

Fixes #24189

Change-Id: I03e6a1ab74fe30322ce6bcfd3d6c42130b6d61be
Reviewed-on: https://go-review.googlesource.com/97819
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_linux_arm64.go

@@ -74,8 +74,11 @@ type sigset_t struct {
 //sys	pselect(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timespec, sigmask *sigset_t) (n int, err error) = SYS_PSELECT6
 
 func Select(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (n int, err error) {
-	ts := Timespec{Sec: timeout.Sec, Nsec: timeout.Usec * 1000}
-	return pselect(nfd, r, w, e, &ts, nil)
+	var ts *Timespec
+	if timeout != nil {
+		ts = &Timespec{Sec: timeout.Sec, Nsec: timeout.Usec * 1000}
+	}
+	return pselect(nfd, r, w, e, ts, nil)
 }
 
 //sysnb	Gettimeofday(tv *Timeval) (err error)

src/syscall/syscall_linux_mips64x.go

@@ -65,8 +65,11 @@ type sigset_t struct {
 //sys	pselect(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timespec, sigmask *sigset_t) (n int, err error) = SYS_PSELECT6
 
 func Select(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (n int, err error) {
-	ts := Timespec{Sec: timeout.Sec, Nsec: timeout.Usec * 1000}
-	return pselect(nfd, r, w, e, &ts, nil)
+	var ts *Timespec
+	if timeout != nil {
+		ts = &Timespec{Sec: timeout.Sec, Nsec: timeout.Usec * 1000}
+	}
+	return pselect(nfd, r, w, e, ts, nil)
 }
 
 //sysnb	Gettimeofday(tv *Timeval) (err error)