Fixes dropping empty BlobVec (#2295)

When dropping the data, we originally only checked the size of an individual item instead of the size of the allocation. However with a capacity of 0, we attempt to deallocate a pointer which was not the result of allocation. That is, an item of `Layout { size_: 8, align_: 8 }` produces an array of `Layout { size_: 0, align_: 8 }` when `capacity = 0`.

Fixes #2294

crates/bevy_ecs/src/storage/blob_vec.rs

@@ -199,13 +199,11 @@ impl BlobVec {
 impl Drop for BlobVec {
  fn drop(&mut self) {
  self.clear();
- if self.item_layout.size() > 0 {
+ let array_layout =
+ array_layout(&self.item_layout, self.capacity).expect("array layout should be valid");
+ if array_layout.size() > 0 {
  unsafe {
- std::alloc::dealloc(
- self.get_ptr().as_ptr(),
- array_layout(&self.item_layout, self.capacity)
- .expect("array layout should be valid"),
- );
+ std::alloc::dealloc(self.get_ptr().as_ptr(), array_layout);
  std::alloc::dealloc(self.swap_scratch.as_ptr(), self.item_layout);
  }
  }
@@ -388,4 +386,11 @@ mod tests {
 
  assert_eq!(*drop_counter.borrow(), 6);
  }
+
+ #[test]
+ fn blob_vec_drop_empty_capacity() {
+ let item_layout = Layout::new::<Foo>();
+ let drop = TypeInfo::drop_ptr::<Foo>;
+ let _ = BlobVec::new(item_layout, drop, 0);
+ }
 }