don't read beyond end of buffer for strlen(cache_name) > 31

* eio_ttc.c (eio_cache_rec_fill): Ensure that rec->cr_name is
always NUL-terminated.  It would lack a trailing NUL whenever
strlen(dmc->cache_name) > CACHE_NAME_LEN.  The length of the
cache_name string may be up to DEV_PATHLEN-1 (127), while the
shorter destination buffer has length CACHE_NAME_SZ, 32.
Each subsequent use of that cr_name member requires that it be
NUL-terminated, so any use would read beyond end of buffer.

Signed-off-by: Jim Meyering <meyering@fb.com>