Add TODO notice and reference to issue

src/base/default_allocator.rs

@@ -46,6 +46,7 @@ where
 
     #[inline]
     unsafe fn allocate_uninitialized(_: R, _: C) -> Self::Buffer {
+        // TODO: Undefined behavior, see #556
         mem::MaybeUninit::<Self::Buffer>::uninit().assume_init()
     }