Crash while running tests for eth/downloader

[piersy]

Description

This happened once, I can't reproduce and since I was not running tests with verbose output I'm not sure which test caused it.

go test -race -count 1 ./eth/downloader
...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4879971]

goroutine 653690 [running]:
github.com/celo-org/celo-blockchain/eth/downloader.(*queue).DeliverBodies.func2(0x0, 0x0)
	/Users/piersy/projects/celo-blockchain/eth/downloader/queue.go:805 +0xb1
github.com/celo-org/celo-blockchain/eth/downloader.(*queue).deliver(0xc00940e3c0, {0x4d74a93, 0x4}, 0x0?, 0xc00c532080, 0xc00debfea0?, {0x4eb1a70, 0x5e16a68}, 0xd, 0xc0040dd8c0, ...)
	/Users/piersy/projects/celo-blockchain/eth/downloader/queue.go:882 +0x7fa
github.com/celo-org/celo-blockchain/eth/downloader.(*queue).DeliverBodies(0xc00940e3c0, {0x4d74a93, 0x4}, {0xc00dfb8000, 0xd, 0xd}, {0xc01171f0a0, 0xd, 0xd}, {0xc01171f110, ...})
	/Users/piersy/projects/celo-blockchain/eth/downloader/queue.go:810 +0x3d5
github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).fetchBodies.func1({0x4ead130?, 0xc027f42900})
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:1411 +0x191
github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).fetchParts(0xc00a6f4000, 0xc0208064e0, 0xc0040dde50, 0xc00940c150, 0xc0040dde40, 0xc0040ddea0, 0xc0040dde90, 0xc0040dde80, 0x0, 0x4dc0ff0, ...)
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:1501 +0x3b5
github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).fetchBodies(0xc00a6f4000, 0x489a7b9?)
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:1418 +0x351
github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).syncWithPeer.func4()
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:584 +0x45
github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).spawnSync.func1()
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:607 +0x95
created by github.com/celo-org/celo-blockchain/eth/downloader.(*Downloader).spawnSync
	/Users/piersy/projects/celo-blockchain/eth/downloader/downloader.go:607 +0x85
FAIL	github.com/celo-org/celo-blockchain/eth/downloader	65.940s
FAIL