efibootmgr: fix potential endless loop with -v

I observed the problem on a system with fairly old and, apparently,
buggy EFI implementation.  A list of boot devices had an invalid
trailing entry.  efidp_size() for that entry returned zero, which means
that the code got stuck looping on that entry.

usr.sbin/efibootmgr/efibootmgr.c

@@ -790,6 +790,8 @@ print_loadopt_str(uint8_t *data, size_t datalen)
 	 */
 	indent = 1;
 	while (dp < edp) {
+		if (efidp_size(dp) == 0)
+			break;
 		efidp_format_device_path(buf, sizeof(buf), dp,
 		    (intptr_t)(void *)edp - (intptr_t)(void *)dp);
 		printf("%*s%s\n", indent, "", buf);