Fix #794. Only remove transactions from memory pool when they're actually in the memory pool.

src/main.cpp

@@ -617,13 +617,17 @@ bool CTransaction::RemoveFromMemoryPool()
 {
     // Remove transaction from memory pool
     CRITICAL_BLOCK(cs_mapTransactions)
+    {
+        uint256 hash = GetHash();
+        if (mapTransactions.count(hash))
         {
             BOOST_FOREACH(const CTxIn& txin, vin)
                 mapNextTx.erase(txin.prevout);
-        mapTransactions.erase(GetHash());
+            mapTransactions.erase(hash);
             nTransactionsUpdated++;
             --nPooledTx;
         }
+    }
     return true;
 }