s3/lib: don't write to buffer (which might be NULL) if bufsize <=0

s3/lib: don't write to buffer (which might be NULL) if bufsize <=0

Some code depends that tdb_pack[va] will return the bytes it would
write to 'buf' if the bufsize passed in is <=0, writing to the
buffer is protected by with lines like
   if (bufsize && bufsize >= len) {
      /* write to 'buf' */
   }

however in these instances the local pointer to the buffer is still
modified
   buf += len;

It's quite probable if bufsize == 0 that buf itself is NULL,
in this case we should protect against performing pointer arithmetic.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>