rpc: avoid crashing on clique getSigner during sync

[holiman]

With an empty database, before this PR:

> clique.getSigner("latest")
ERROR[10-29|13:43:56.110] RPC method clique_getSigner crashed: runtime error: invalid memory address or nil pointer dereference
goroutine 153 [running]:
github.com/ethereum/go-ethereum/rpc.(*callback).call.func1()
	github.com/ethereum/go-ethereum/rpc/service.go:200 +0x89
panic({0x103e200, 0x1b263a0})
	runtime/panic.go:1038 +0x215
github.com/ethereum/go-ethereum/consensus/clique.ecrecover(0x0, 0x2)
	github.com/ethereum/go-ethereum/consensus/clique/clique.go:153 +0x170
github.com/ethereum/go-ethereum/consensus/clique.(*Clique).Author(...)
	github.com/ethereum/go-ethereum/consensus/clique/clique.go:213
github.com/ethereum/go-ethereum/consensus/clique.(*API).GetSigner(0xc003f7e3f0, 0x18)
	github.com/ethereum/go-ethereum/consensus/clique/api.go:217 +0x11d
reflect.Value.call({0xc0045ff560, 0xc002d1dc78, 0x79af2f9ab3c8}, {0x118fbee, 0x4}, {0xc004000eb0, 0x2, 0xc00401c640})
	reflect/value.go:543 +0x814

With this PR:

> clique.getSigner("latest")
WARN [10-29|13:48:50.806] Served clique_getSigner                  reqid=9 t="278.136µs" err="missing block -1"
Error: missing block -1
	at web3.js:6357:37(47)
	at send (web3.js:5091:62(35))
	at <eval>:1:17(4)

Q.E.D