Bugfix: Initialize CWallet::nOrderPosNext on an empty wallet, and save it in db

src/walletdb.h

@@ -115,6 +115,12 @@ public:
         return Read(std::string("bestblock"), locator);
     }
 
+    bool WriteOrderPosNext(int64 nOrderPosNext)
+    {
+        nWalletDBUpdated++;
+        return Write(std::string("orderposnext"), nOrderPosNext);
+    }
+
     bool ReadDefaultKey(std::vector<unsigned char>& vchPubKey)
     {
         vchPubKey.clear();