Fix a race that fake execution client sends event to closed channel.
Fix a race that fake execution client sends event to closed channel.

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

containerd/containerd,w9n/cri-containerd,zhangzhenhao/cri-containerd,dmcgowan/containerd,mikebrow/cri-containerd,mikebrow/cri-containerd,zhangzhenhao/cri-containerd,w9n/cri-containerd,thaJeztah/containerd,mikebrow/containerd,thaJeztah/containerd,containerd/containerd,zhangzhenhao/cri-containerd,w9n/cri-containerd,mikebrow/cri-containerd,estesp/containerd,mikebrow/containerd,vdemeester/containerd,vdemeester/containerd,dmcgowan/containerd,estesp/containerd