Merge bitcoin#26172: p2p: ProcessHeadersMessage(): fix received_new_header

bdcafb9 p2p: ProcessHeadersMessage(): fix received_new_header (Larry Ruane)

Pull request description:

  Follow-up to bitcoin#25717. The commit "Utilize anti-DoS headers download strategy" changed how this bool variable is computed, so that its value is now the opposite of what it should be.

  Prior to bitcoin#25717:
  ```
  bool received_new_header{WITH_LOCK(::cs_main, return m_chainman.m_blockman.LookupBlockIndex(headers.back().GetHash()) == nullptr)};
  ```
  After bitcoin#25717 (simplified):
  ```
  {
      LOCK(cs_main);
      last_received_header = m_chainman.m_blockman.LookupBlockIndex(headers.back().GetHash());
  }
  bool received_new_header{last_received_header != nullptr};
  ```

ACKs for top commit:
  dergoegge:
    ACK bdcafb9
  glozow:
    ACK bdcafb9, I believe this is correct and don't see anything to suggest the switch was intentional.
  stickies-v:
    ACK bdcafb9

Tree-SHA512: 35c12762f1429585a0b1c15053e310e83efb28c3d8cbf4092fad9fe81c893f6d766df1f2b20624882acb9654d0539a0c871f587d7090dc2a198115adf59db3ec

Author: glozow

src/net_processing.cpp

@@ -2846,7 +2846,7 @@ void PeerManagerImpl::ProcessHeadersMessage(CNode& pfrom, Peer& peer,
 
     // If we don't have the last header, then this peer will have given us
     // something new (if these headers are valid).
-    bool received_new_header{last_received_header != nullptr};
+    bool received_new_header{last_received_header == nullptr};
 
     // Now process all the headers.
     BlockValidationState state;