> I don't know of a platform where a dereferencing of NULL (which happens in a separate translation unit, which is already compiled and linked, and will not be subject to LTO and other optimisations) within the various read/write/seek/tell functions doesn't result in an immediate crash.

Although in a very different content, I have seen "dereferencing" a null pointer in C++ not crash immediately, if you dereference it to call a nonvirtual class member function, e.g,

    t->foo();

Depending on how this gets compiled and the implementation of `foo()`, the segfault may not come at the line above, where technically `t` is being dereferenced. It may come inside `foo`, or somewhere further down the call chain. The resulting crash may not even manifest as a segfault.