windows: errnoErr(): return ENOENT to reduce code duplication

Change-Id: I51c08a4add9c2b31c4bb7c4421dde227b4fff489
GitHub-Last-Rev: ef8ef8646ca90df893bee4cc8d7879dc1a80ebe4
GitHub-Pull-Request: golang/sys#89
Reviewed-on: https://go-review.googlesource.com/c/sys/+/259537
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Alex Brainman <alex.brainman@gmail.com>

windows/mkwinsyscall/mkwinsyscall.go

@@ -303,17 +303,13 @@ func (r *Rets) SetReturnValuesCode() string {
 
 func (r *Rets) useLongHandleErrorCode(retvar string) string {
 	const code = `if %s {
-		if e1 != 0 {
-			err = errnoErr(e1)
-		} else {
-			err = %sEINVAL
-		}
+		err = errnoErr(e1)
 	}`
 	cond := retvar + " == 0"
 	if r.FailCond != "" {
 		cond = strings.Replace(r.FailCond, "failretval", retvar, 1)
 	}
-	return fmt.Sprintf(code, cond, syscalldot())
+	return fmt.Sprintf(code, cond)
 }
 
 // SetErrorCode returns source code that sets return parameters.
@@ -867,7 +863,7 @@ var (
 func errnoErr(e {{syscalldot}}Errno) error {
 	switch e {
 	case 0:
-		return nil
+		return {{syscalldot}}EINVAL
 	case errnoERROR_IO_PENDING:
 		return errERROR_IO_PENDING
 	}