Skip nil validator for snapshot

[hyeonLewis]

Proposed changes

This PR fixed not to touch nil validator pointer if found.

The valSet in snapshot might contain demoted proposers if the CN's total staking amount became lower than 5M KAIA but not removed from validators explicitly.

Even before Kaia HF, potential nil dereference pointer must be prevented.

Below is an example of such error

ERROR[09/25,20:21:12 +09] [26] Proposer is not available now.            proposer address=0x993efC86499FF9a47E5770040a02F37ed9469911
ERROR[09/25,20:21:12 +09] [39] RPC method kaia_getCouncil crashed: runtime error: invalid memory address or nil pointer dereference
goroutine 434555796 [running]:
github.com/kaiachain/kaia/networks/rpc.(*callback).call.func1()
        /home/ubuntu/kaia/networks/rpc/service.go:225 +0x85
panic({0x1ee0f00?, 0x4c7d340?})
        /usr/local/go/src/runtime/panic.go:770 +0x132
github.com/kaiachain/kaia/consensus/istanbul/validator.RecoverWeightedCouncilProposer({0x3a62340?, 0xc0ae32f680}, {0xc05082f900?, 0x22, 0x2d?})
        /home/ubuntu/kaia/consensus/istanbul/validator/weighted.go:138 +0x2a1
github.com/kaiachain/kaia/consensus/istanbul/backend.(*Snapshot).UnmarshalJSON(0xc0efb7a400, {0xc0d4b25980, 0x188b, 0x1980})
        /home/ubuntu/kaia/consensus/istanbul/backend/snapshot.go:378 +0x28b

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Further comments

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[ian0371]

Let's merge tomorrow after checking with ollie

[yoomee1313]

cc. @blukat29