internal/poll: ensure that newPoolPipe doesn't return a nil pointer

The function could occasionally return a nil pointer as a non-nil
interface, confusing the calling code.

Fixes #45520

Change-Id: Ifd35613728efa2cee9903177e85d369155074804
Reviewed-on: https://go-review.googlesource.com/c/go/+/309429


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Andy Pan <panjf2000@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>