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.

(cherry picked from commit bf87d4a4bfaa86e97079754e93fe14595adf07c5)