The timeout parameter might be nil, don't dereference it
unconditionally.

CL 97819 did the same for the syscall package.

Updates golang/go#24189

Change-Id: I95a93468c7d8431abf2e9a3a9b8d0fbd1a223e0d
Reviewed-on: https://go-review.googlesource.com/97820
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>