we don't need zero-sized freeze-sensitive visiting

Author: RalfJung

src/helpers.rs

@@ -58,6 +58,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             .map(|(size, _)| size)
             .unwrap_or_else(|| place.layout.size)
         );
+        assert!(size.bytes() > 0);
         // Store how far we proceeded into the place so far. Everything to the left of
         // this offset has already been handled, in the sense that the frozen parts
         // have had `action` called on them.