Fix issue 4470, memcached caching causing a SEGV because the wrong
type is returned.

* subversion/libsvn_subr/cache-memcache.c
  (memcache_get): Return data wrapped in svn_stringbuf_t rather than
   svn_string_t.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570642 13f79535-47bb-0310-9956-ffa450edef68