libct: State: ensure Resources is not nil

Since opencontainers/cgroups v0.0.2 (commit b206a015), all stuct
Resources fields are annotated with "omitempty" attribute.
As a result, the loaded configuration may have Resources == nil.

It is totally OK (rootless containers may have no resources configured)
except since commit 6c5441e5, cgroup v1 fs manager requires Resources to
be set in the call to NewManager (this is a cgroup v1 deficiency,
or maybe our implementation deficiency, or both).

To work around this, let's add code to ensure Resources is never nil
after loading from state.json.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/factory_linux.go

@@ -164,6 +164,10 @@ func loadState(root string) (*State, error) {
 	if err := json.NewDecoder(f).Decode(&state); err != nil {
 		return nil, err
 	}
+	// Cgroup v1 fs manager expect Resources to never be nil.
+	if state.Config.Cgroups.Resources == nil {
+		state.Config.Cgroups.Resources = &cgroups.Resources{}
+	}
 	return state, nil
 }