syscall: fix ForkLock spurious close(0) on pipe failure

Pipe (and therefore forkLockPipe) does not make any guarantees
about the state of p after a failed Pipe(p). Avoid that assumption
and the too-clever goto, so that we don't accidentally Close a real fd
if the failed pipe leaves p[0] or p[1] set >= 0.

Fixes #50057
Fixes CVE-2021-44717

Change-Id: Iff8e19a6efbba0c73cc8b13ecfae381c87600bb4
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1291270


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/370576


Run-TryBot: Filippo Valsorda <filippo@golang.org>
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>