openpgp-tool: fix buffer overflow on serials with MSb set

Fixes the following crash:
  $ openpgp-tool --card-info
  Using reader with a card: Linux Foundation Multifunction Composite Gadget - vincent [python-usb-f-ccid] 00 00
  AID:             d2:76:00:01:24:01:03:41:ff:65:a4:9b:68:64:00:00
  Version:         3.41
  Manufacturer:    unmanaged S/N range
  *** buffer overflow detected ***: terminated
  Abandon (core dumped)
"a4:9b:68:64" from the AID being the serial, and:
  (gdb) print (data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3])
  $12 = -1533319068
but
  (gdb) print (unsigned long) (data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3])
  $13 = 18446744072176232548

Avoid the shifts and cast altogether.

src/tools/openpgp-tool.c

@@ -325,9 +325,7 @@ static char *prettify_serialnumber(u8 *data, size_t length)
 {
  if (data != NULL && length >= 4) {
  static char result[15]; /* large enough for even 2*3 digits + separator */
- unsigned long serial = (unsigned long) (data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3]);
-
- sprintf(result, "%08lX", serial);
+ sprintf(result, "%02X%02X%02X%02X", data[0], data[1], data[2], data[3]);
  return result;
  }
  return NULL;