Make nStartingHeight atomic

Author: TheBlueMatt

src/net.h

@@ -628,7 +628,7 @@ class CNode
 
 public:
     uint256 hashContinue;
-    int nStartingHeight;
+    std::atomic<int> nStartingHeight;
 
     // flood relay
     std::vector<CAddress> vAddrToSend;