On 32 bit systems, integer underflows can cause the membuffer usage counters
to grow beyond 32 bit limits.  That makes all entries appear to be "small"
and being kept instead of being potentially evicted.

* subversion/libsvn_subr/cache-membuffer.c
  (svn_membuffer_t): Update comment.
  (membuffer_cache_set_internal): Adding a cast to prevent off-by-4GB in the
                                  cache->DATA_USED tracker in 32 bits systems.

Found by: vitalif{_AT_}yourcmc.ru
Suggested by: James McCoy <jamessan{_AT_}debian.org>


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567985 13f79535-47bb-0310-9956-ffa450edef68