[scudo] Avoid empty statement warnings

An empty macro that expands to just `... else ;` can get
warnings from some compilers (e.g. GCC's -Wempty-body).

Reviewed By: cryptoad, vitalybuka

Differential Revision: https://reviews.llvm.org/D100693

GitOrigin-RevId: d9b2641aa5f350f3aafcb61782ef372712159be9
1 file changed