p2p: ProcessHeadersMessage(): fix received_new_header

Follow-up to #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.

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;