Fix potential containerd panic during graceful shutdown.
Fix potential containerd panic during graceful shutdown.

Signed-off-by: Lantao Liu <e12a959b3f33bba53f1a03da59760f37bd7265d7@google.com>

vdemeester/containerd,mikebrow/containerd,thaJeztah/containerd,mikebrow/containerd,dmcgowan/containerd,containerd/containerd,vdemeester/containerd,dmcgowan/containerd,crosbymichael/containerd,containerd/containerd,crosbymichael/containerd,thaJeztah/containerd,estesp/containerd,estesp/containerd