Pruning proceeds ahead of last sync blocks

[AlexeyAkhunov]

When syncing to Ropsten, where it takes a long time for sync to start, I observed that pruning has started ahead of sync:

INFO [10-16|10:46:00.988] Initialising Ethereum protocol           versions="[63 62]" network=3
INFO [10-16|10:46:01.010] Loaded most recent local header          number=0 hash=419410…ca4a2d td=1048576 age=50y6mo5d
INFO [10-16|10:46:01.011] Loaded most recent local full block      number=0 hash=419410…ca4a2d td=1048576 age=50y6mo5d
INFO [10-16|10:46:01.011] Loaded most recent local fast block      number=0 hash=419410…ca4a2d td=1048576 age=50y6mo5d
INFO [10-16|10:46:01.011] Pruner started 
INFO [10-16|10:46:01.011] Loaded local transaction journal         transactions=0 dropped=0
INFO [10-16|10:46:01.012] Regenerated local transaction journal    transactions=0 accounts=0
INFO [10-16|10:46:01.012] Initialising Firehose protocol           versions=[1]
INFO [10-16|10:46:01.088] New local node record                    seq=23 id=a3c8dc9a49dedf3b ip=127.0.0.1 udp=30303 tcp=30303
INFO [10-16|10:46:01.088] Started P2P networking                   self=enode://d3228a32673a4c6ec8556a6796bf3a4ea94d288aed9db43a98d7ba742abd296624b2c7f02bd3ad88250ce4b74a777dfdb30fef0a4cad7f484b8c748221f6c626@127.0.0.1:30303
INFO [10-16|10:46:01.091] IPC endpoint opened                      url=/Users/alexeyakhunov/Library/Ethereum/testnet/geth.ipc
INFO [10-16|10:51:01.036] Save last pruned block num               num=3000
INFO [10-16|10:56:01.028] Save last pruned block num               num=6000
INFO [10-16|11:01:01.016] Save last pruned block num               num=9000
INFO [10-16|11:06:01.007] Save last pruned block num               num=12000

The reason is underflow when computing diff in this code:

func calculateNumOfPrunedBlocks(curentBlock, lastPrunedBlock uint64, blocksBeforePruning uint64, blocksBatch uint64) (uint64, uint64, bool) {
	diff := curentBlock - lastPrunedBlock - blocksBeforePruning
	switch {
	case diff >= blocksBatch:
		return lastPrunedBlock, lastPrunedBlock + blocksBatch, true
	case diff > 0 && diff < blocksBatch:
		return lastPrunedBlock, lastPrunedBlock + diff, true
	default:
		return lastPrunedBlock, lastPrunedBlock, false
	}
}

Also there is a typo in the word curentBlock :)