site: Initialise st->scratch with SETUP_BUFFER_LEN space.

Otherwise, if we're unlucky, we end up trying to construt a PROD
packet before we have ever used buffer_copy on scratch, which causes
an assertion failure ("buf->size <= buf->len - amount" at util.c:257).

Reported-by: Simon Tatham <anakin@pobox.com>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>