les: fix crash happening after setHead

[holiman]

This PR fixes a crash that happened on one of the sealers, after a setHead was performed on it:

 Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth panic: runtime error: invalid memory address or nil pointer dereference
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth [signal SIGSEGV: segmentation violation code=0x1 addr=0x1c0 pc=0xc69da5]
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth goroutine 155 [running]:
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth github.com/ethereum/go-ethereum/les.(*serverHandler).broadcastLoop(0xc007bf98c0)
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth github.com/ethereum/go-ethereum/les/server_handler.go:424 +0x3a5
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth created by github.com/ethereum/go-ethereum/les.(*serverHandler).start
Jan 04 10:10:20 rinkeby-aws-eu-north-1-001 geth github.com/ethereum/go-ethereum/les/server_handler.go:92 +0x6f