ethstats: prevent panic if head block is not available

[rjl493456442]

This pull request fixes a flaw in ethstats.

The panic happens when the local blockchain is reorging which causes the original head block not reachable(number->hash canonical mapping is deleted). In order to prevent the panic, the block nilness will be checked in ethstats.

INFO [02-19|01:42:43.476] Chain reorg detected                     number=19,258,596 hash=9d98f1..e6fc8a drop=1 dropfrom=9e5d7e..e9aea2 add=1 addfrom=5c3ee3..9b5f97
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x100bdfa]

goroutine 4464 [running]:
github.com/ethereum/go-ethereum/core/types.(*Block).Header(...)
	github.com/ethereum/go-ethereum/core/types/block.go:356
github.com/ethereum/go-ethereum/ethstats.(*Service).assembleBlockStats(0xc000368360, 0x0)
	github.com/ethereum/go-ethereum/ethstats/ethstats.go:645 +0xba
github.com/ethereum/go-ethereum/ethstats.(*Service).reportBlock(0xc000368360, 0xc0b21b25b0?, 0xc0d185d330?)
	github.com/ethereum/go-ethereum/ethstats/ethstats.go:612 +0x25
github.com/ethereum/go-ethereum/ethstats.(*Service).report(0xc0161cbe00?, 0xc0161cbcbc?)
	github.com/ethereum/go-ethereum/ethstats/ethstats.go:527 +0x3b
github.com/ethereum/go-ethereum/ethstats.(*Service).loop(0xc000368360, 0xc00085d3e0, 0xc00085d4a0)
	github.com/ethereum/go-ethereum/ethstats/ethstats.go:334 +0x9bf
created by github.com/ethereum/go-ethereum/ethstats.(*Service).Start in goroutine 1
	github.com/ethereum/go-ethereum/ethstats/ethstats.go:210 +0x127

[holiman]

LGTM