cipd: fix panic in deployerImpl.packagePath
cipd: fix panic in deployerImpl.packagePath

Check if the description returned by readDescription is not nil.

R=iannucci@chromium.org, vadimsh@chromium.org
BUG=688918

Review-Url: https://codereview.chromium.org/2677053002

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