Fix bug 4367: correctly detect auth_challenge cells we can't use

Found by frosty_un, bugfix on 0.2.3.6-alpha, fix suggested by arma.

changes/bug4367

@@ -0,0 +1,5 @@
+  o Minor bugfixes:
+    - Successfully detect AUTH_CHALLENGE cells with no recognized
+      authentication type listed. Fixes bug 4367; bugfix on 0.2.3.6-alpha.
+      Found by frosty_un.
+

src/or/command.c

@@ -1100,7 +1100,7 @@ command_process_auth_challenge_cell(var_cell_t *cell, or_connection_t *conn)
 
   conn->handshake_state->received_auth_challenge = 1;
 
-  if (use_type && public_server_mode(get_options())) {
+  if (use_type >= 0 && public_server_mode(get_options())) {
     log_info(LD_OR, "Got an AUTH_CHALLENGE cell from %s:%d: Sending "
              "authentication",
              safe_str(conn->_base.address), conn->_base.port);