fix failure in TestProposerFrequency (#3293)
fix failure in TestProposerFrequency (#3293)

```
--- FAIL: TestProposerFrequency (2.50s)

panic: empty validator set [recovered]

        panic: empty validator set



goroutine 91 [running]:

testing.tRunner.func1(0xc000a98c00)

        /usr/local/go/src/testing/testing.go:792 +0x6a7

panic(0xeae7e0, 0x11fbb30)

        /usr/local/go/src/runtime/panic.go:513 +0x1b9

github.com/tendermint/tendermint/types.(*ValidatorSet).RescalePriorities(0xc0000e7380, 0x0)

        /go/src/github.com/tendermint/tendermint/types/validator_set.go:106 +0x1ac

github.com/tendermint/tendermint/state.TestProposerFrequency(0xc000a98c00)

        /go/src/github.com/tendermint/tendermint/state/state_test.go:335 +0xb44

testing.tRunner(0xc000a98c00, 0x111a4d8)

        /usr/local/go/src/testing/testing.go:827 +0x163

created by testing.(*T).Run

        /usr/local/go/src/testing/testing.go:878 +0x65a

FAIL    github.com/tendermint/tendermint/state  3.139s
```
tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint