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

Author: thebluefish

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);
+    }
 }