Bug fix: SC panic when received instance DELETE event.

[little-cui]

No description provided.

[coveralls]

Coverage decreased (-0.05%) to 71.614% when pulling 09c4db0 on little-cui:bugfix into fcc2d45 on ServiceComb:master.

[codecov-io]

Codecov Report

Merging #207 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #207   +/-   ##
=======================================
  Coverage   67.92%   67.92%           
=======================================
  Files          17       17           
  Lines        3588     3588           
=======================================
  Hits         2437     2437           
  Misses        978      978           
  Partials      173      173

Impacted Files	Coverage Δ
server/service/instances.go	70.9% <0%> (-0.17%)	⬇️
server/service/tag.go	74.41% <0%> (+0.58%)	⬆️

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[little-cui]

#321