windows: don't check non-existent return code in GetStartupInfo

Same as CL 520275 did in package syscall.

For golang/go#31316

Change-Id: Ie9d8fed7f40b9e562534d5e91488b4ba1ac44f34
Reviewed-on: https://go-review.googlesource.com/c/sys/+/520295
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

windows/syscall_windows_test.go

@@ -1173,3 +1173,12 @@ func TestTimePeriod(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestGetStartupInfo(t *testing.T) {
+	var si windows.StartupInfo
+	err := windows.GetStartupInfo(&si)
+	if err != nil {
+		// see https://go.dev/issue/31316
+		t.Fatalf("GetStartupInfo: got error %v, want nil", err)
+	}
+}