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 <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: oiooj

src/internal/poll/fd.go

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