IMPORT: cebtree: Replace offset calculation with offsetof to avoid UB

IMPORT: cebtree: Replace offset calculation with offsetof to avoid UB

This is the same as the equivalent fix in ebtree:

The C standard specifies that it's undefined behavior to dereference
NULL (even if you use & right after). The hand-rolled offsetof idiom
&(((s*)NULL)->f) is thus technically undefined. This clutters the
output of UBSan and is simple to fix: just use the real offsetof when
it's available.

This is cebtree commit 2d08958858c2b8a1da880061aed941324e20e748.