Type checking in SKIP_BACK macro

struct align_probe { char x; long int y; };

#define OFFSETOF(s, i) ((size_t) &((s *)0)->i)

#define SKIP_BACK(s, i, p) ((s *)((char *)p - OFFSETOF(s, i)))

#define SKIP_BACK(s, i, p) ({ s *_ptr = ((s *)((char *)p - OFFSETOF(s, i))); ASSERT_DIE(&_ptr->i == p); _ptr; })

#define BIRD_ALIGN(s, a) (((s)+a-1)&~(a-1))

#define CPU_STRUCT_ALIGN (sizeof(struct align_probe))