examples/chat_server bug

[1u0]

When client disconnects, chat server doesn't remove session_ptr from m_connections (chat.cpp, line #79):

send_to_all(encode_message("server",m_connections[client]+" has left the chat."));

this makes server crazy, after sending messages from other clients.

The following logic should fix this:

std::cout << "client " << client << " left the lobby." << std::endl;

const std::string alias = it->second;
m_connections.erase(it);

// send user list and signoff message to all clients
send_to_all(serialize_state());
send_to_all(encode_message("server", alias+" has left the chat."));