Fix Visual Studio 2019 warnings

[elfenix]

This changes fixes sevearl warnings triggered by Visual Studio
2019. These are largerly due to 64bit size_t and extended static
analysis provided by newer revisions of intellisense. Instead
of using %lu, use %zu for size_t variables. Add a couple casts
where required. Finally, fix an issue where intellisense gets
tripped up by not considering that two null compared pointers would
be equivalent.

[elfenix]

This is a ping PR, if anyone is still watching this repository I'd be willing to clean this up.

I'm using fct.h in one of my projects, and thought this might be useful. This isn't really cleaned up for "production" distribution, but if there's ANY interest in this at all, I'd be willing to fix issues. This does introduce a minimum req of Visual Studio at 2013 - for %zu - if earlier revisions are required - could potentially resolve using casts or macros. Also, default VS2019 config won't compile and run unit tests as warnings will trigger compile errors.