server: free strduped port numbers

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: I3a7a9c8e8dc7557aa51d0b9caa244537e5e7007d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4410
Tested-by: jenkins

src/server/telnet_server.c

@@ -719,3 +719,8 @@ int telnet_register_commands(struct command_context *cmd_ctx)
 	telnet_port = strdup("4444");
 	return register_commands(cmd_ctx, NULL, telnet_command_handlers);
 }
+
+void telnet_service_free(void)
+{
+	free(telnet_port);
+}