Use size_t for buffer sizes in SetHints.c

These variables store values returned from strlen() as a size_t
and are passed to Xmalloc, which expects a size_t, so lets stop
converting back and forth to int along the way.

Reported by: Konstantin SKliarov
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>