internal/syscall/unix: allow calling getrandom(..., 0, ...)

Calling getrandom() with a zero length is actually valid and useful:

- Calling getrandom(..., 0, 0) will block until the RNG is initialized.
- Calling getrandom(..., 0, GRND_NONBLOCK) will query whether the RNG
  is initialized.

So instead of short circuiting execution for these circumstances, pass
this through to the syscall.

Change-Id: I15178f087908a2d8be6c020a1ef800cc0a074742
Reviewed-on: https://go-review.googlesource.com/c/go/+/615315
Run-TryBot: Jason Donenfeld <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Auto-Submit: Jason Donenfeld <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: zx2c4

src/internal/syscall/unix/getrandom.go

@@ -19,14 +19,11 @@ type GetRandomFlag uintptr
 
 // GetRandom calls the getrandom system call.
 func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
-	if len(p) == 0 {
-		return 0, nil
-	}
 	if getrandomUnsupported.Load() {
 		return 0, syscall.ENOSYS
 	}
 	r1, _, errno := syscall.Syscall(getrandomTrap,
-		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(unsafe.Pointer(unsafe.SliceData(p))),
 		uintptr(len(p)),
 		uintptr(flags))
 	if errno != 0 {