gh-123022: Fix crash with `Py_Initialize` in background thread (#123052)

Check that the current default heap is initialized in
`_mi_os_get_aligned_hint` and `mi_os_claim_huge_pages`.

The mimalloc function `_mi_os_get_aligned_hint` assumes that there is an
initialized default heap. This is true for our main thread, but not for
background threads. The problematic code path is usually called during
initialization (i.e., `Py_Initialize`), but it may also be called if the
program allocates large amounts of memory in total.

The crash only affected the free-threaded build.