treewide nit: avoid NULL arithmetics

treewide nit: avoid NULL arithmetics

(u)intptr_t casts seem the best in terms of compliance:
https://stackoverflow.com/q/45220134/587396

Otherwise with clang 18 we can get warnings like
 ../$path:$line:$col: runtime error: applying non-zero offset $num to null pointer
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../$path:$line:$col