Fixed segfault causes by coins view

src/darksend.cpp

@@ -2206,6 +2206,13 @@ void ThreadCheckDarkSendPool()
         darkSendPool.CheckTimeout();
 
         if(c % 60 == 0){
+            LOCK(cs_main);
+            /*
+                cs_main is required for doing masternode.Check because something
+                is modifying the coins view without a mempool lock. It causes
+                segfaults from this code without the cs_main lock.
+            */
+
             vector<CMasterNode>::iterator it = vecMasternodes.begin();
             //check them separately
             while(it != vecMasternodes.end()){