Fix crash: null socket check in Connection::write()

During header sync, peers disconnect (EOF) asynchronously. If a
chain tip changed handler fires request_full_block() after the
peer disconnected but before the connection was removed, writing
to the null m_socket caused SIGSEGV (signal 11).

Add null guard in Connection::write() — skip if socket is gone.

Author: frstrtr

src/impl/ltc/coin/p2p_connection.hpp

@@ -64,6 +64,7 @@ class Connection
 
     void write(std::unique_ptr<RawMessage>& rmsg)
     {
+        if (!m_socket) return;  // peer disconnected
         m_socket->write(std::move(rmsg));
     }