Fix crashing bug caused by orphan(s) with duplicate prevout.hash

Rebased-From: def2fdb
Rebased-By: Wladimir J. van der Laan

src/main.cpp

@@ -324,6 +324,8 @@ void static EraseOrphanTx(uint256 hash)
     BOOST_FOREACH(const CTxIn& txin, it->second.vin)
     {
         map<uint256, set<uint256> >::iterator itPrev = mapOrphanTransactionsByPrev.find(txin.prevout.hash);
+        if (itPrev == mapOrphanTransactionsByPrev.end())
+            continue;
         itPrev->second.erase(hash);
         if (itPrev->second.empty())
             mapOrphanTransactionsByPrev.erase(itPrev);