Merge bitcoin/bitcoin#24976: netgroup: Follow-up for #22910

e5d1831 [netgroup] Use nStartByte as offset for the last byte of the group (dergoegge)

Pull request description:

  This addresses my review [comments](bitcoin/bitcoin#22910 (comment)) I left on #22910.

  This has no effect on the current logic as `nStartByte` is only used for internal addresses which only ever add 10 whole bytes to the returned group. However to avoid future bugs, I think we should use `nStartByte` as offset for the last byte as well, in case we ever add a new address type that makes makes use of `nStartByte` and adds fractional bytes to the group.

ACKs for top commit:
  jnewbery:
    Code review ACK e5d1831
  theStack:
    Concept and code-review ACK e5d1831

Tree-SHA512: 4c08c7d6cb38b553e998798b3e3b790177aaa2141a48e277dfd538e01a7fccadf644329e93c5b0fb5e7e4037494c8dfe061b94eb52c6b31dc21bdf99eb0e311a

Author: fanquake

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;