Fix a membuffer cache bug introduced in 1.8:  Trying to make room
for the new contents of an existing key / item may remove the old
contents and, thus, the old index entry.  Before manually removing
the entry, we must check whether it still exists.

* subversion/libsvn_subr/cache-membuffer.c
  (membuffer_cache_set_internal): re-do entry lookup before removal

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442053 13f79535-47bb-0310-9956-ffa450edef68