Protecting from use of NULL pointer in ExitInstance; don't try to use…

[rgd]

Hello Kazuhiro-san! I've used XKeymacs for years and really like it. I just found your fork and made my own fork to try building it. It built fine for me in VS2017, but I ran into a minor issue which I fixed and I thought I'd offer it to you in a pull request and to introduce myself and let you know your fork is appreciated.

Regards,

Rob Davenport