syscall: fix ComputerName on Windows

GetComputerName expects n to be the size of the buffer, and
on output contains the number of characters copied to the buffer.

CL 493036 broke ComputerName by always setting n to 0.

Change-Id: I3f4b30d2f9825d321a6d28ec82bdc7b6294e04e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/499035
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/syscall/syscall_windows.go

@@ -585,8 +585,8 @@ func Rename(oldpath, newpath string) (err error) {
 }
 
 func ComputerName() (name string, err error) {
-	b := make([]uint16, MAX_COMPUTERNAME_LENGTH+1)
-	var n uint32
+	var n uint32 = MAX_COMPUTERNAME_LENGTH + 1
+	b := make([]uint16, n)
 	e := GetComputerName(&b[0], &n)
 	if e != nil {
 		return "", e

src/syscall/syscall_windows_test.go

@@ -37,6 +37,16 @@ func TestOpen_Dir(t *testing.T) {
 	}
 }
 
+func TestComputerName(t *testing.T) {
+	name, err := syscall.ComputerName()
+	if err != nil {
+		t.Fatalf("ComputerName failed: %v", err)
+	}
+	if len(name) == 0 {
+		t.Error("ComputerName returned empty string")
+	}
+}
+
 func TestWin32finddata(t *testing.T) {
 	dir := t.TempDir()