libefivar: Fix AcpiEx print logic

Add logic that checks if the code doesn't overflow
ACPI_EXTENDED_HID_DEVICE_PATH node when searching for optional
strings. If the string is not provided in the device path node
default value of "\0" is used.

Upstream PR: https://bugzilla.tianocore.org/show_bug.cgi?id=4555
Obtained from: https://github.com/tianocore/edk2/commit/96ed60dfd7d4818a532216d64ee87b13fae5c726

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1388