Check if precompute seal option exists

plus indentation style fix

libethcore/EthashSealEngine.cpp

@@ -54,7 +54,8 @@ void EthashSealEngine::generateSeal(BlockInfo const& _bi)
 	m_farm.setWork(m_sealing);
 	m_farm.start(m_sealer);
 	m_farm.setWork(m_sealing);		// TODO: take out one before or one after...
-	if (option("precomputeDAG")[0] == 1)
+	bytes shouldPrecompute = option("precomputeDAG");
+	if (!shouldPrecompute.empty() && shouldPrecompute[0] == 1)
 		Ethash::ensurePrecomputed((unsigned)_bi.number());
 }
 

libethereum/Client.h

@@ -140,8 +140,8 @@ public:
 	bool turboMining() const { return m_turboMining; }
 	/// Enable/disable GPU mining.
 	void setTurboMining(bool _enable = true) { m_turboMining = _enable; if (isMining()) startMining(); }
-    /// Enable/disable precomputing of the DAG for next epoch
-    void setShouldPrecomputeDAG(bool _precompute);
+	/// Enable/disable precomputing of the DAG for next epoch
+	void setShouldPrecomputeDAG(bool _precompute);
 
 	/// Check to see if we'd mine on an apparently bad chain.
 	bool mineOnBadChain() const { return m_mineOnBadChain; }