milo: fix panic in swarming.getSwarming
milo: fix panic in swarming.getSwarming

check resError before dereferncing sr

R=hinoka@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2113003002

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