Fix bug when leap array as token bucket(sample bucket is 1)

[louyuting]

Describe what this PR does / why we need it

Benchmark_SlowRequestRatio_SlotCheck_4-32     	{"timestamp":"2020-11-21 07:11:18.18210","caller":"circuit_breaker.go:345","logLevel":"ERROR","msg":"Failed to get current bucket in slowRequestLeapArray.currentCounter()"}
Provided time timeMillis=1605942677000 is already behind old.BucketStart=1605942678000. now=1605942677999
github.com/alibaba/sentinel-golang/core/stat/base.(*LeapArray).currentBucketOfTime
	/home/ytlou/share/golang/sentinel-golang/core/stat/base/leap_array.go:204
github.com/alibaba/sentinel-golang/core/stat/base.(*LeapArray).CurrentBucket
	/home/ytlou/share/golang/sentinel-golang/core/stat/base/leap_array.go:164
github.com/alibaba/sentinel-golang/core/circuitbreaker.(*slowRequestLeapArray).currentCounter
	/home/ytlou/share/golang/sentinel-golang/core/circuitbreaker/circuit_breaker.go:343
github.com/alibaba/sentinel-golang/core/circuitbreaker.(*slowRtCircuitBreaker).OnRequestComplete
	/home/ytlou/share/golang/sentinel-golang/core/circuitbreaker/circuit_breaker.go:257
github.com/alibaba/sentinel-golang/core/circuitbreaker.(*MetricStatSlot).OnCompleted
	/home/ytlou/share/golang/sentinel-golang/core/circuitbreaker/stat_slot.go:44
github.com/alibaba/sentinel-golang/core/base.(*SlotChain).exit
	/home/ytlou/share/golang/sentinel-golang/core/base/slot_chain.go:278
github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit.func1
	/home/ytlou/share/golang/sentinel-golang/core/base/entry.go:92
sync.(*Once).doSlow
	/home/ytlou/gosdk/go/src/sync/once.go:66
sync.(*Once).Do
	/home/ytlou/gosdk/go/src/sync/once.go:57
github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit
	/home/ytlou/share/golang/sentinel-golang/core/base/entry.go:77
command-line-arguments.doCheck
	/home/ytlou/share/golang/sentinel-golang/tests/benchmark/circuitbreaker/circuitbreaker_benchmark_test.go:19
command-line-arguments.Benchmark_SlowRequestRatio_SlotCheck_4.func1
	/home/ytlou/share/golang/sentinel-golang/tests/benchmark/circuitbreaker/circuitbreaker_benchmark_test.go:60
testing.(*B).RunParallel.func1
	/home/ytlou/gosdk/go/src/testing/benchmark.go:768
runtime.goexit
	/home/ytlou/gosdk/go/src/runtime/asm_amd64.s:1374
{"timestamp":"2020-11-21 07:11:18.18210","caller":"entry.go:80","logLevel":"ERROR","msg":"Sentinel internal panic in SentinelEntry.Exit()"}
runtime error: invalid memory address or nil pointer dereference
github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit.func1.1
	/home/ytlou/share/golang/sentinel-golang/core/base/entry.go:80
runtime.gopanic
	/home/ytlou/gosdk/go/src/runtime/panic.go:969
runtime.panicmem
	/home/ytlou/gosdk/go/src/runtime/panic.go:212
runtime.sigpanic
	/home/ytlou/gosdk/go/src/runtime/signal_unix.go:742
github.com/alibaba/sentinel-golang/core/circuitbreaker.(*slowRtCircuitBreaker).OnRequestComplete
	/home/ytlou/share/golang/sentinel-golang/core/circuitbreaker/circuit_breaker.go:261
github.com/alibaba/sentinel-golang/core/circuitbreaker.(*MetricStatSlot).OnCompleted
	/home/ytlou/share/golang/sentinel-golang/core/circuitbreaker/stat_slot.go:44
github.com/alibaba/sentinel-golang/core/base.(*SlotChain).exit
	/home/ytlou/share/golang/sentinel-golang/core/base/slot_chain.go:278
github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit.func1
	/home/ytlou/share/golang/sentinel-golang/core/base/entry.go:92
sync.(*Once).doSlow
	/home/ytlou/gosdk/go/src/sync/once.go:66
sync.(*Once).Do
	/home/ytlou/gosdk/go/src/sync/once.go:57
github.com/alibaba/sentinel-golang/core/base.(*SentinelEntry).Exit
	/home/ytlou/share/golang/sentinel-golang/core/base/entry.go:77
command-line-arguments.doCheck
	/home/ytlou/share/golang/sentinel-golang/tests/benchmark/circuitbreaker/circuitbreaker_benchmark_test.go:19
command-line-arguments.Benchmark_SlowRequestRatio_SlotCheck_4.func1
	/home/ytlou/share/golang/sentinel-golang/tests/benchmark/circuitbreaker/circuitbreaker_benchmark_test.go:60
testing.(*B).RunParallel.func1
	/home/ytlou/gosdk/go/src/testing/benchmark.go:768
runtime.goexit
	/home/ytlou/gosdk/go/src/runtime/asm_amd64.s:1374

Does this pull request fix one issue?

Describe how you did it

If the sample count is 1 in leapArray, we guarantee currentBucketOfTime returns the bucket.

Describe how to verify it

Special notes for reviews

[codecov-io]

Codecov Report

Merging #327 (2d11791) into master (8d33c7c) will decrease coverage by 0.07%.
The diff coverage is 18.51%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   50.91%   50.83%   -0.08%     
==========================================
  Files          68       68              
  Lines        3814     3820       +6     
==========================================
  Hits         1942     1942              
- Misses       1606     1609       +3     
- Partials      266      269       +3     

Impacted Files	Coverage Δ
core/stat/base/leap_array.go	70.40% <0.00%> (-1.47%)	⬇️
core/circuitbreaker/circuit_breaker.go	67.22% <20.00%> (-0.93%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8d33c7c...2d11791. Read the comment docs.

[luckyxiaoqiang]

LGTM.

[sczyh30]

We may refine the currentCounter method of xxxLeapArray in circuit breakers: return error directly and let callers handle it (instead of just returning nil).

[sczyh30]

LGTM