Fix subscript[0] bug in net.cpp if GetGroup returns a 0-sized vector

src/net.cpp

@@ -2876,5 +2876,5 @@ uint64_t CConnman::CalculateKeyedNetGroup(const CAddress& ad) const
 {
     std::vector<unsigned char> vchNetGroup(ad.GetGroup());
 
-    return GetDeterministicRandomizer(RANDOMIZER_ID_NETGROUP).Write(&vchNetGroup[0], vchNetGroup.size()).Finalize();
+    return GetDeterministicRandomizer(RANDOMIZER_ID_NETGROUP).Write(vchNetGroup.data(), vchNetGroup.size()).Finalize();
 }