Security fix for Linux Port

I just uploaded a security fix for the SDL frontend of the Linux Port, I advise users that rely on it’s security to upgrade.

The Details

If a mode change command was sent with a log2bpp value other than 3, 4 or 5 a structure describing the screen memory would be freed and the pointer left dangling. In my experience it then crashes, but it might be possible for a RISC OS program to cause other actions with the permissions of the SDL frontend.