s3: fix crash when using --s3-download-url after migration to SDKv2

Before this change rclone was crashing when the download URL did not
supply an X-Amz-Storage-Class header.

This change allows the header to be missing.

See: https://forum.rclone.org/t/sigsegv-on-ubuntu-24-04/48047

backend/s3/s3.go

@@ -5829,7 +5829,7 @@ func (o *Object) downloadFromURL(ctx context.Context, bucketPath string, options
 		ContentEncoding:    header("Content-Encoding"),
 		ContentLanguage:    header("Content-Language"),
 		ContentType:        header("Content-Type"),
-		StorageClass:       types.StorageClass(*header("X-Amz-Storage-Class")),
+		StorageClass:       types.StorageClass(deref(header("X-Amz-Storage-Class"))),
 	}
 	o.setMetaData(&head)
 	return resp.Body, err