cmd/streamdata: check against overwriting

This prevents inadvertently queuing an existing vod, only to fail the check after copy completes.

Signed-off-by: Yonah <contrib@gensokyo.uk>

cmd/streamdata/main.go

@@ -112,6 +112,16 @@ func main() {
 				return err
 			}
 
+			var e *streamdata.VOD
+			if e, err = channel.Load(&ident); err != nil {
+				if !errors.Is(err, os.ErrNotExist) {
+					return err
+				}
+			} else {
+				printVOD(e)
+				return errors.New("attempting to overwrite existing vod")
+			}
+
 			return channel.Add(&ident, func(
 				v *streamdata.VOD,
 				w io.Writer,