uxnds can crash on stack overflow

[danstowell]

Hi - I believe that uxnds 0.5.2 can crash on a stack overflow, even though "uxn stacks are circular" (which I think means they shouldn't overflow).

The issue was found using the "linf_land" demo in this gist, which worked fine on the desktop SDL VM. On a Nintendo DSi XL running uxnds or uxnds_debug, the demo crashed after about 30 seconds with "Guru Meditation Error". That's with the January version of my code (the first commit to that gist). The current version of that gist doesn't crash, and the only difference is to add a single POP which fixes my bug of having one leftover value per screen-refresh. (There was a single #01 left on the stack each time.)

(Discussion on Mastodon here)