libcontainer: intelrdt: fix a GetStats() issue

This fixes a GetStats() issue introduced in opencontainers#1590:
If Intel RDT is enabled by hardware and kernel, but intelRdt is not
specified in original config, GetStats() will return error unexpectedly
because we haven't called Apply() to create intelrdt group or attach
tasks for this container. As a result, runc events command will have no
output.

Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>

libcontainer/intelrdt/intelrdt.go

@@ -443,6 +443,11 @@ func (m *IntelRdtManager) GetPath() string {
 
 // Returns statistics for Intel RDT
 func (m *IntelRdtManager) GetStats() (*Stats, error) {
+	// If intelRdt is not specified in config
+	if m.Config.IntelRdt == nil {
+		return nil, nil
+	}
+
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	stats := NewStats()