backport r1604598 from trunk

    apr_skiplist becomes corrupt when nodes are reused.

    Submitted By: Takashi Sato <takashi tks st>, covener
    Committed By: covener



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.6.x@1605761 13f79535-47bb-0310-9956-ffa450edef68