prebuffer: Remove buffer tree node on errors.

prebuffer: Remove buffer tree node on errors.

When the ->post_select method of a filter returns negative, it *must*
remove its buffer tree node as well. The prebuffer missed to do that,
which results in a stale reference to the buffer tree which keeps
the audiod slot busy. When no more slots are free, audiod hangs or
exits with a "no more free slots" message. This patch avoids this.