unix, plan9: avoid writing to p when Pipe(p) fails

Generally speaking Go functions make no guarantees
about what has happened to result parameters on error,
and Pipe is no exception: callers should avoid looking at
p if Pipe returns an error.

However, we had a bug in which ForkExec was using the
content of p after a failed Pipe, and others may too.
As a robustness fix, make Pipe avoid writing to p on failure.

windows.Pipe already avoided writing to p on failure.

For golang/go#50057.

Change-Id: I93ed06b06a9981793c119c1d7df689fbe79b4116
Reviewed-on: https://go-review.googlesource.com/c/sys/+/370614
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/syscall_darwin.go

@@ -159,8 +159,10 @@ func Pipe(p []int) (err error) {
 	}
 	var x [2]int32
 	err = pipe(&x)
-	p[0] = int(x[0])
-	p[1] = int(x[1])
+	if err == nil {
+		p[0] = int(x[0])
+		p[1] = int(x[1])
+	}
 	return
 }