internal/poll: cast off the last reference of SplicePipe in test

Updates #45059

Change-Id: I9f377abcc7b77136ae6cf4896b968f73c758b559
Reviewed-on: https://go-review.googlesource.com/c/go/+/317510
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/internal/poll/splice_linux_test.go

@@ -46,6 +46,7 @@ func TestSplicePipePool(t *testing.T) {
 		poll.PutPipe(p)
 	}
 	ps = nil
+	p = nil
 
 	// Exploit the timeout of "go test" as a timer for the subsequent verification.
 	timeout := 5 * time.Minute