channel: Fix use after free in channel_do_open_actions()

Fortunately, our tor_free() is setting the variable to NULL after so we were
in a situation where NULL was always used instead of the transport name.

This first appeared in 894ff2dc84 and results in
basically no bridge with a transport being able to use DoS defenses.

Fixes #40345

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket40345

@@ -0,0 +1,5 @@
+  o Minor bugfixes (channel, DoS):
+    - Fix a possible non fatal assertion BUG() due to a too early free of a
+      string when noting down the client connection for the DoS defenses
+      subsystem. Fixes bug 40345; bugfix on 0.4.3.4-rc
+

src/core/or/channel.c

@@ -1887,11 +1887,11 @@ channel_do_open_actions(channel_t *chan)
         geoip_note_client_seen(GEOIP_CLIENT_CONNECT,
                                &remote_addr, transport_name,
                                now);
-        tor_free(transport_name);
         /* Notify the DoS subsystem of a new client. */
         if (tlschan && tlschan->conn) {
           dos_new_client_conn(tlschan->conn, transport_name);
         }
+        tor_free(transport_name);
       }
       /* Otherwise the underlying transport can't tell us this, so skip it */
     }