[zbin] Fix size used for memset in alloc_output_file

The output->buf field is a pointer, not an array, so sizeof() is not
applicable.  We must use the allocated string length instead.

Identified by gcc:

  util/zbin.c: In function ‘alloc_output_file’:
  util/zbin.c:146:37: warning: argument to ‘sizeof’ in ‘memset’ call
    is the same expression as the destination; did you mean to
    dereference it? [-Wsizeof-pointer-memaccess]
  memset ( output->buf, 0xff, sizeof ( output->buf ) );

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>