Fix array-bounds compiler warning on gcc11+ in list.h (#580)

Fix array-bounds compiler warning on gcc11+ in list.h (#580)

listGET_OWNER_OF_NEXT_ENTRY computes `( pxConstList )->pxIndex->pxNext` after
verifying that `( pxConstList )->pxIndex` points to `xListEnd`, which due to
being a MiniListItem_t, can be shorter than a ListItem_t. Thus,
`( pxConstList )->pxIndex` is a `ListItem_t *` that extends past the end of the
`List_t` whose `xListEnd` it points to. This is fixed by accessing `pxNext`
through a `MiniListItem_t` instead.