s3blob: fix data race (#3480)

w.pr cannot be used to indicate that the pipe has been closed. The problem with
using w.pr is that a write to it in the goroutine launched by writer.open can
race with a read on w.pr in Close.

We just don't write to w.pr in writer.open. The read end of the pipe can still be
closed twice, but that's fine.

blob/s3blob/s3blob.go

@@ -380,7 +380,6 @@ func (w *writer) open(r io.Reader, closePipeOnError bool) {
 		if err != nil {
 			if closePipeOnError {
 				w.pr.CloseWithError(err)
-				w.pr = nil
 			}
 			w.err = err
 		}