grub: add a fix for malformed device path handling

grub: add a fix for malformed device path handling

This change fixes the malformed device paths in EFI handling.
Device paths of length 4 or shorter could cause different
kinds of unexpected behaviours.

This patch is NOT a part of [1], but is a dependency of one
of the patches included in the series.

[1] https://lists.gnu.org/archive/html/grub-devel/2021-03/msg00007.html

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>