  o Minor fixes (security):
    - Make memwipe() do nothing when passed a NULL pointer
      or zero size. Check size argument to memwipe() for underflow.
      Closes bug #18089. Reported by "gk", patch by "teor".
      Bugfix on 0.2.3.25 and 0.2.4.6-alpha (#7352),
      commit 49dd5ef3 on 7 Nov 2012.
