[3.13] gh-118561: Fix crash involving list.extend in free-threaded build (GH-118723) (#118863)

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.
(cherry picked from commit 2402715e10d00ef60fad2948d8461559d084eb36)

Co-authored-by: Sam Gross <colesbury@gmail.com>