Fix crash when mining with empty keypool.

Since the introduction of the ScriptForMining callback, the mining
functions (setgenerate and generate) crash with an assertion failure
(due to a NULL pointer script returned) if the keypool is empty.  Fix
this by giving a proper error.

src/miner.cpp

@@ -444,8 +444,10 @@ void static BitcoinMiner(const CChainParams& chainparams)
     GetMainSignals().ScriptForMining(coinbaseScript);
 
     try {
-        //throw an error if no script was provided
-        if (!coinbaseScript->reserveScript.size())
+        // Throw an error if no script was provided.  This can happen
+        // due to some internal error but also if the keypool is empty.
+        // In the latter case, already the pointer is NULL.
+        if (!coinbaseScript || coinbaseScript->reserveScript.empty())
             throw std::runtime_error("No coinbase script available (mining requires a wallet)");
 
         while (true) {