os/exec: do not close pipes on a double-Start error

os/exec: do not close pipes on a double-Start error

This fixes a bug introduced in CL 401834 in which calling Start twice
with pipes attached to a command would spuriously close those pipes
when returning the error from the second Start call.

For #50436.

Change-Id: I3563cc99c0a0987752190fef95da3e9927a76fda
Reviewed-on: https://go-review.googlesource.com/c/go/+/436095
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>