fix(serverHandler/upload): stop reading next part if error occured

0 parts will make loop endless, just break it if error occured.

When this happened, an error "multipart: NextPart: EOF" which wrapps
`io.EOF` is returned. Although we can use `errors.Is(err, io.EOF)` to
detect, we need to prevent doing this currently, since `errors.Is` was
introduced in Go 1.13 which does not support Windows XP platform.

Author: marjune163

src/serverHandler/upload.go

@@ -44,12 +44,11 @@ func (h *handler) saveUploadFiles(fsPrefix string, overwriteExists bool, r *http
 
 	for {
 		part, err := reader.NextPart()
-		if err == io.EOF {
-			break
-		}
 		if err != nil {
-			errs = append(errs, err)
-			continue
+			if err != io.EOF {
+				errs = append(errs, err)
+			}
+			break
 		}
 
 		filename := part.FileName()