Fix SF bug 574207 (chained __slots__ dealloc segfault).

This is inspired by SF patch 581742 (by Jonathan Hogg, who also
submitted the bug report, and two other suggested patches), but
separates the non-GC case from the GC case to avoid testing for GC
several times.

Had to fix an assert() from call_finalizer() that asserted that the
object wasn't untracked, because it's possible that the object isn't
GC'ed!