[netgroup] Use nStartByte as offset for the last byte of the group

Should we ever introduce a new address type that makes use of
`nStartByte` and adds fractional bytes to the group, then nStartByte
should be used as the offset for the last byte.

src/netgroup.cpp

@@ -71,7 +71,7 @@ std::vector<unsigned char> NetGroupManager::GetGroup(const CNetAddr& address) co
     // ...for the last byte, push nBits and for the rest of the byte push 1's
     if (nBits > 0) {
         assert(num_bytes < addr_bytes.size());
-        vchRet.push_back(addr_bytes[num_bytes] | ((1 << (8 - nBits)) - 1));
+        vchRet.push_back(addr_bytes[num_bytes + nStartByte] | ((1 << (8 - nBits)) - 1));
     }
 
     return vchRet;