Fix segfault if prpc.Client is using nil Options.
Fix segfault if prpc.Client is using nil Options.

R=iannucci@chromium.org, dnj@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2675803005

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go