Added checks for null pointers in Shutdown

Estetics

src/init.cpp

@@ -78,7 +78,9 @@ void Shutdown(void* parg)
         StopNode();
         {
             LOCK(cs_main);
+            if (pcoinsTip)
                 pcoinsTip->Flush();
+            if (pblocktree)
                 pblocktree->Flush();
             delete pcoinsTip;
             delete pcoinsdbview;