In addition to what others have said, which basically boils down to "it'll never return an error… unless it does", there's also the aspect where if you start as

In addition to what others have said, which basically boils down to "it'll never return an error… unless it does", there's also the aspect where if you start assuming things about the kernel environment you're not coding in "POSIX C99" like the title says, but to "Debian GNU/Linux circa 2020". And that's part of the "and do it well". A thousand years from now, if the C99&POSIX spec survives, will support correctly written code. Dereferencing a null pointer can also be a security issue (mostly for kernels, though) so one should assume a malicious general environment.