Faster mining, minor fix.

alethzero/MainWin.cpp

@@ -845,7 +845,7 @@ void Main::refreshBlockChain()
 	string filter = ui->blockChainFilter->text().toLower().toStdString();
 	auto const& bc = m_client->blockChain();
 	unsigned i = (ui->showAll->isChecked() || !filter.empty()) ? (unsigned)-1 : 10;
-	for (auto h = bc.currentHash(); h != bc.genesisHash() && i; h = bc.details(h).parent, --i)
+	for (auto h = bc.currentHash(); h != bc.genesisHash() && bc.details(h) && i; h = bc.details(h).parent, --i)
 	{
 		auto d = bc.details(h);
 		auto bm = blockMatch(filter, d, h, bc);

libethcore/BlockInfo.cpp

@@ -30,7 +30,7 @@
 using namespace std;
 using namespace eth;
 
-u256 eth::c_genesisDifficulty = (u256)1 << 22;
+u256 eth::c_genesisDifficulty = (u256)1 << 12;
 
 BlockInfo::BlockInfo(): timestamp(Invalid256)
 {