[Go] Address Gosec vulnerabilities in Go client mustache template (#12540)

* Fix potential file inclusion via variable

* Fix deferring unsafe method "Close" on type "*os.File"

* Update samples

* Correction to use existing variable

* Correction generated samples

samples/openapi3/client/petstore/go/go-petstore/client.go

@@ -430,11 +430,14 @@ func (c *APIClient) decode(v interface{}, b []byte, contentType string) (err err
 
 // Add a file to the multipart request
 func addFile(w *multipart.Writer, fieldName, path string) error {
-	file, err := os.Open(path)
+    file, err := os.Open(filepath.Clean(path))
 	if err != nil {
 		return err
 	}
-	defer file.Close()
+    err = file.Close()
+    if err != nil {
+        return err
+    }
 
 	part, err := w.CreateFormFile(fieldName, filepath.Base(path))
 	if err != nil {