Minor optimization in membuffer cache:  we allocate space to items
at a 16 byte granularity.  Thus, we can replace contents as long
as their aligned size does not increase.

* subversion/libsvn_subr/cache-membuffer.c
  (membuffer_cache_set_internal): improve check whether we actually
   need to re-allocate space within the cache

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442056 13f79535-47bb-0310-9956-ffa450edef68