Fix UB/data-race in RPCNotifyBlockChange

ActiveTip() is *not* thread-safe, as the required ::cs_main lock will be
released as ActiveChainstate() returns.

ActiveTip() is an alias for ActiveChainstate().m_chain.Tip(), so m_chain
may be involved in a data-race (UB).

src/init.cpp

@@ -1751,12 +1751,12 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // At this point, the RPC is "started", but still in warmup, which means it
     // cannot yet be called. Before we make it callable, we need to make sure
     // that the RPC's view of the best block is valid and consistent with
-    // ChainstateManager's ActiveTip.
+    // ChainstateManager's active tip.
     //
     // If we do not do this, RPC's view of the best block will be height=0 and
     // hash=0x0. This will lead to erroroneous responses for things like
     // waitforblockheight.
-    RPCNotifyBlockChange(chainman.ActiveTip());
+    RPCNotifyBlockChange(WITH_LOCK(chainman.GetMutex(), return chainman.ActiveTip()));
     SetRPCWarmupFinished();
 
     uiInterface.InitMessage(_("Done loading").translated);