Fix crash-at-shutdown if exiting before initializing wallet

src/init.cpp

@@ -100,6 +100,7 @@ void Shutdown()
     StopNode();
     {
         LOCK(cs_main);
+        if (pwalletMain)
             pwalletMain->SetBestChain(CBlockLocator(pindexBest));
         if (pblocktree)
             pblocktree->Flush();