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

Fixes #24189

Change-Id: I03e6a1ab74fe30322ce6bcfd3d6c42130b6d61be
Reviewed-on: https://go-review.googlesource.com/97819
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>