onedrive: fix spurious "Couldn't decode error response: EOF" DEBUG

This DEBUG was being generated on redirects which don't have a JSON
body and is irrelevant.

backend/onedrive/onedrive.go

@@ -942,7 +942,8 @@ func errorHandler(resp *http.Response) error {
 	// Decode error response
 	errResponse := new(api.Error)
 	err := rest.DecodeJSON(resp, &errResponse)
-	if err != nil {
+	// Redirects have no body so don't report an error
+	if err != nil && resp.Header.Get("Location") == "" {
 		fs.Debugf(nil, "Couldn't decode error response: %v", err)
 	}
 	if errResponse.ErrorInfo.Code == "" {