Only assign c.Context if err == nil.
Only assign c.Context if err == nil.

This prevents an unrecoverble panic if Authenticate returns an error (since
c.Context is used to do e.g. logging later in the call).

R=nodir@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2210023003

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