checker: fix rule checker panic (#2160)

[sre-bot]

cherry-pick #2160 to release-3.1

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Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

when no more store to replace, it panics:

[2020/02/26 11:46:45.244 +08:00] [FATAL] [log.go:294] [panic] [recover="\"invalid memory address or nil pointer dereference\""] [stack="github.com/pingcap/log.Fatal\n\t/home/birdstorm/go/pkg/mod/github.com/pingcap/log@v0.0.0-2020011      7041106-d28c14d3b1cd/global.go:59\ngithub.com/pingcap/pd/v3/pkg/logutil.LogPanic\n\t/home/birdstorm/go/src/github.com/pingcap/pd/pkg/logutil/log.go:294\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:679\nruntime.panicmem\n\t      /usr/local/go/src/runtime/panic.go:199\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_unix.go:394\ngithub.com/pingcap/kvproto/pkg/metapb.(*Store).GetId\n\t/home/birdstorm/go/pkg/mod/github.com/pingcap/kvproto@v0.0.0-202002130      74014-83e827908584/pkg/metapb/metapb.pb.go:221\ngithub.com/pingcap/pd/v3/server/core.DistinctScore\n\t/home/birdstorm/go/src/github.com/pingcap/pd/server/core/store.go:453\ngithub.com/pingcap/pd/v3/server/schedule/checker.(*RuleChec      ker).fixBetterLocation\n\t/home/birdstorm/go/src/github.com/pingcap/pd/server/schedule/checker/rule_checker.go:187\ngithub.com/pingcap/pd/v3/server/schedule/checker.(*RuleChecker).fixRulePeer\n\t/home/birdstorm/go/src/github.com/pin      gcap/pd/server/schedule/checker/rule_checker.go:110\ngithub.com/pingcap/pd/v3/server/schedule/checker.(*RuleChecker).Check\n\t/home/birdstorm/go/src/github.com/pingcap/pd/server/schedule/checker/rule_checker.go:59\ngithub.com/pingca      p/pd/v3/server/schedule.(*CheckerController).CheckRegion\n\t/home/birdstorm/go/src/github.com/pingcap/pd/server/schedule/checker_controller.go:58\ngithub.com/pingcap/pd/v3/server/cluster.(*coordinator).patrolRegions\n\t/home/birdsto      rm/go/src/github.com/pingcap/pd/server/cluster/coordinator.go:125"]

What is changed and how it works?

• check nil
• add unit test

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch (release-3.1)

[codecov-io]

Codecov Report

Merging #2161 into release-3.1 will decrease coverage by 0.07%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release-3.1    #2161      +/-   ##
===============================================
- Coverage        74.94%   74.87%   -0.08%     
===============================================
  Files              194      194              
  Lines            19982    19985       +3     
===============================================
- Hits             14975    14963      -12     
- Misses            3875     3891      +16     
+ Partials          1132     1131       -1

Impacted Files	Coverage Δ
server/schedule/checker/rule_checker.go	76.07% <100%> (+0.44%)	⬆️
server/region_syncer/client.go	81.03% <0%> (-3.45%)	⬇️
server/kv/etcd_kv.go	72.72% <0%> (-2.6%)	⬇️
server/core/storage.go	71.85% <0%> (-1.01%)	⬇️
server/config/option.go	87.44% <0%> (-0.87%)	⬇️
server/grpc_service.go	56.65% <0%> (-0.86%)	⬇️
pkg/btree/btree.go	86.84% <0%> (-0.81%)	⬇️
server/server.go	64.32% <0%> (-0.72%)	⬇️
server/handler.go	51.61% <0%> (+0.43%)	⬆️
server/member/leader.go	76.53% <0%> (+1.02%)	⬆️
... and 2 more

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[rleungx]

LGTM

[nolouch]

/merge

[sre-bot]

/run-all-tests