FIX: potential use-after-free in tls_proxy

If the TLS session to the client was established (when tls_session_activated()
was called), and the connection to the server was also established successfully
(ec in onConnect() callback was not set); but -- in the mean time -- the this-
pointer was deallocated via std::enable_shared_from_this, we end up in a use-
after free situation.

This sporadically apeeared in CI but wasn't reproducible locally, see randombit#4112.

src/cli/tls_proxy.cpp

@@ -282,13 +282,18 @@ class tls_proxy_session final : public std::enable_shared_from_this<tls_proxy_se
       }
 
       void tls_session_activated() override {
-         auto onConnect = [this](boost::system::error_code ec, const tcp::resolver::iterator& /*endpoint*/) {
+         auto onConnect = [self = weak_from_this()](boost::system::error_code ec,
+                                                    const tcp::resolver::iterator& /*endpoint*/) {
             if(ec) {
                log_error("Server connection", ec);
                return;
             }
-            server_read(boost::system::error_code(), 0);  // start read loop
-            proxy_write_to_server({});
+            if(self.expired()) {
+               log_error("Server connection established, but client session already closed");
+               return;
+            }
+            self.lock()->server_read(boost::system::error_code(), 0);  // start read loop
+            self.lock()->proxy_write_to_server({});
          };
          async_connect(m_server_socket, m_server_endpoints, onConnect);
       }