gh-118561: Fix crash involving list.extend in free-threaded build (#118723)

gh-118561: Fix crash involving list.extend in free-threaded build (#118723)

The `list_preallocate_exact` function did not zero initialize array
contents. In the free-threaded build, this could expose uninitialized
memory to concurrent readers between the call to
`list_preallocate_exact` and the filling of the array contents with
items.