[build] Fix "'%s' directive argument is null" error

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>

src/interface/efi/efi_debug.c

 
 	/* Sanity check */
 	if ( ! handle ) {
-		printf ( "HANDLE %s could not retrieve protocols\n",
-			 efi_handle_name ( handle ) );
+		printf ( "HANDLE %p could not retrieve protocols\n", handle );
 		return;
 	}