Fix: issue #951 etcd exit panic

[watermelo]

What this PR does:
Fix issue #951

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #1013 (dd1b395) into develop (0cb6edb) will decrease coverage by 0.05%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           develop    #1013      +/-   ##
===========================================
- Coverage    59.66%   59.60%   -0.06%     
===========================================
  Files          261      261              
  Lines        12919    12921       +2     
===========================================
- Hits          7708     7702       -6     
- Misses        4240     4248       +8     
  Partials       971      971              

Impacted Files	Coverage Δ
filter/filter_impl/sentinel_filter.go	63.04% <ø> (ø)
registry/nacos/registry.go	50.36% <57.14%> (-0.77%)	⬇️
registry/etcdv3/registry.go	64.15% <75.00%> (-0.77%)	⬇️
registry/etcdv3/listener.go	67.50% <100.00%> (+1.71%)	⬆️
remoting/etcdv3/listener.go	43.00% <100.00%> (ø)
registry/kubernetes/registry.go	51.11% <0.00%> (-8.89%)	⬇️
cluster/cluster_impl/failback_cluster_invoker.go	75.90% <0.00%> (+2.40%)	⬆️

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[wenxuwan]

LGTM