Minor membuffer cache optimization.

* subversion/libsvn_subr/cache-membuffer.c
  (svn_membuffer_cache_get,
   svn_membuffer_cache_get_partial): no need to clear our scratch pool
   here because we don't use it in the getters

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442054 13f79535-47bb-0310-9956-ffa450edef68