checker: fix rule checker panic

[disksing]

Signed-off-by: disksing [email protected]

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/[email protected]      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/[email protected]      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 #2160 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2160      +/-   ##
==========================================
+ Coverage   76.06%   76.12%   +0.06%     
==========================================
  Files         195      195              
  Lines       20577    20580       +3     
==========================================
+ Hits        15651    15667      +16     
+ Misses       3733     3724       -9     
+ Partials     1193     1189       -4

Impacted Files	Coverage Δ
server/schedule/checker/rule_checker.go	76.07% <100%> (+0.44%)	⬆️
pkg/etcdutil/etcdutil.go	82.6% <0%> (-8.7%)	⬇️
server/region_syncer/client.go	81.03% <0%> (-3.45%)	⬇️
server/tso/tso.go	77.37% <0%> (-2.92%)	⬇️
server/member/leader.go	73.1% <0%> (-1.69%)	⬇️
server/server.go	79.39% <0%> (+0.27%)	⬆️
server/grpc_service.go	57.08% <0%> (+0.42%)	⬆️
server/handler.go	51.61% <0%> (+0.43%)	⬆️
server/config/option.go	90.04% <0%> (+0.86%)	⬆️
server/core/storage.go	73.86% <0%> (+1%)	⬆️
... and 4 more

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[rleungx]

LGTM

[disksing]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.1 in PR #2161