[Ur] Help psychoanalyzing some C compilers?

[Adam Chlipala]

Can anyone help figure out what is bothering recent C compilers about 
part of the Ur/Web runtime-system code?  Here's the issue on GitHub:
     https://github.com/urweb/urweb/issues/31

It seems like somehow C compilers are assuming that certain function 
arguments can't be NULL, leading said compilers to report some code as 
dead when it's guarded by tests for non-NULL-ness, but I don't see why.

Thanks!