internal/poll: clear completed Buffers to permit earlier collection

Updates #45163

Change-Id: I73a6f22715550e0e8b83fbd3ebec72ef019f153f
Reviewed-on: https://go-review.googlesource.com/c/go/+/373374
Run-TryBot: Lee Baokun <bk@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/internal/poll/fd.go

@@ -74,6 +74,7 @@ func consume(v *[][]byte, n int64) {
 			return
 		}
 		n -= ln0
+		(*v)[0] = nil
 		*v = (*v)[1:]
 	}
 }