election: fix the panic bug of lease

[JmPotato]

Signed-off-by: JmPotato ghzpotato@gmail.com

What problem does this PR solve?

#4044 makes (*Leadership).Reset() will set its lease to nil, which will cause panic due to the nil pointer when we call some methods of the lease.

What is changed and how it works?

Check lease itself before we deref it.

Check List

Tests

• Unit test
• Integration test

Release note

None.

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[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 9527f1f

[disksing]

/merge

[ti-chi-bot]

@disksing: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[codecov]

Codecov Report

Merging #4061 (9527f1f) into master (9d066b6) will decrease coverage by 0.04%.
The diff coverage is 82.22%.

@@            Coverage Diff             @@
##           master    #4061      +/-   ##
==========================================
- Coverage   74.64%   74.59%   -0.05%     
==========================================
  Files         257      258       +1     
  Lines       25972    26059      +87     
==========================================
+ Hits        19388    19440      +52     
- Misses       4859     4886      +27     
- Partials     1725     1733       +8     

Flag	Coverage Δ
unittests	74.59% <82.22%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/election/lease.go	84.12% <0.00%> (-5.71%)	⬇️
server/core/store.go	78.27% <14.28%> (-1.73%)	⬇️
server/schedule/placement/fit.go	86.06% <62.50%> (-1.87%)	⬇️
server/schedule/placement/region_rule_cache.go	93.87% <93.87%> (ø)
server/schedule/checker/rule_checker.go	80.57% <100.00%> (+0.11%)	⬆️
server/schedule/placement/rule.go	96.22% <100.00%> (+0.07%)	⬆️
server/schedule/placement/rule_manager.go	84.61% <100.00%> (+1.48%)	⬆️
pkg/tempurl/tempurl.go	45.00% <0.00%> (-15.00%)	⬇️
server/schedule/placement/label_constraint.go	86.36% <0.00%> (-9.10%)	⬇️
server/tso/local_allocator.go	71.62% <0.00%> (-6.76%)	⬇️
... and 16 more

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