Fix memcached support for caches using svn_stringbuf_t values.

* subversion/libsvn_subr/cache-memcache.c
  (memcache_get, memcache_set): Account for the svn_stringbuf_t invariant
   s->data[s->len] == '\0' by copying the trailing NUL byte into the cache.
   Fixes checksum mismatch errors seen during 'make check' with memcached.

W: svn: E200014: Checksum mismatch for '/home/stsp/svn/svn-trunk/subversion/tests/cmdline/svn-test-work/working_copies/basic_tests-1/A/B/E/alpha':
W:    expected:  d1fa4a3ced98961674a441930a51f2d3
W:      actual:  440bc1eb6bd3fbeefb383cbaa6b02528


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1585686 13f79535-47bb-0310-9956-ffa450edef68