fix(export-backup): Fix double free in export backup (#7780) (#7783)

Fix issue of double-free in export-backup. kvBuf is released by
the reducer so the Writer shouldn't free it.

(cherry picked from commit 00a67d5)

ee/backup/run.go

@@ -217,11 +217,7 @@ func (bw *bufWriter) Write(buf *z.Buffer) error {
 		}
 		return nil
 	})
-	if err != nil {
-		return err
-	}
-	buf.Release()
-	return nil
+	return errors.Wrap(err, "bufWriter failed to write")
 }
 
 func runExportBackup() error {