Use '%p' directive, and print handle's address if the address is null
and the handle doesn't have a name.  This fixes the following
compilation error:

  interface/efi/efi_debug.c:334:3: error: '%s' directive
  argument is null [-Werror=format-overflow=]

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>