malloc: Preserve arena free list/thread count invariant [BZ #20370]

It is necessary to preserve the invariant that if an arena is
on the free list, it has thread attach count zero.  Otherwise,
when arena_thread_freeres sees the zero attach count, it will
add it, and without the invariant, an arena could get pushed
to the list twice, resulting in a cycle.

One possible execution trace looks like this:

Thread 1 examines free list and observes it as empty.
Thread 2 exits and adds its arena to the free list,
  with attached_threads == 0).
Thread 1 selects this arena in reused_arena (not from the free list).
Thread 1 increments attached_threads and attaches itself.
  (The arena remains on the free list.)
Thread 1 exits, decrements attached_threads,
  and adds the arena to the free list.

The final step creates a cycle in the usual way (by overwriting the
next_free member with the former list head, while there is another
list item pointing to the arena structure).

tst-malloc-thread-exit exhibits this issue, but it was only visible
with a debugger because the incorrect fix in bug 19243 removed
the assert from get_free_list.