never construct value on stack in new_box_zeroed

On lower opt-levels the compiler might not optimize out the
`layout.size() == 0` branch and emits code for the if-body. This will
cause a stack allocation for `Self`. Avoid calling new_zeroed() and
directly construct the Box from a dangling pointer instead.

src/lib.rs

@@ -2128,7 +2128,12 @@ pub unsafe trait FromZeros: TryFromBytes {
         // no allocation, but `Box` does require a correct dangling pointer.
         let layout = Layout::new::<Self>();
         if layout.size() == 0 {
-            return Box::new(Self::new_zeroed());
+            // SAFETY: Contructing a Box to a ZST from a dangling pointer is
+            // explicitly allowed:
+            // https://doc.rust-lang.org/std/boxed/index.html#memory-layout
+            unsafe {
+                return Box::from_raw(NonNull::dangling().as_ptr());
+            }
         }
 
         // TODO(#429): Add a "SAFETY" comment and remove this `allow`.