Fix a grub issue with incorrect values from an usb device. From the official
description from NVD [1]:
During USB device initialization, descriptors are read with very little
bounds checking and assumes the USB device is providing sane values.
If properly exploited, an attacker could trigger memory corruption leading
to arbitrary code execution allowing a bypass of the Secure Boot mechanism.
This patch is a part of a bigger security collection for grub [2].
