Fix crash on exit on Android with destroy_on_exit

[jblazquez]

When mi_option_destroy_on_exit is set, the call to _mi_heap_unsafe_destroy_all (which calls mi_prim_get_default_heap, which in turn calls pthread_getspecific with _mi_heap_default_key) is made after the call to _mi_prim_thread_done_auto_done (which calls pthread_key_delete with _mi_heap_default_key).

It appears that, at least on Android, calling pthread_getspecific after pthread_key_delete returns a null value, which causes mi_prim_get_default_heap to return the empty heap, and this line to crash due to heap->tld being null.

This change moves the call to _mi_prim_thread_done_auto_done after all of the uses of mi_prim_get_default_heap, so accessing the default heap is possible.