Ensure we have a full EFI_DEVICE_PATH header before we try to look at

Ensure we have a full EFI_DEVICE_PATH header before we try to look at
its length. Some BIOSes pad the length of the device path to an even
amount. When we had a device path that was somehow an odd length, we'd
wind up having 1 byte left that we were bogusly interpreting as a full
device path. We'd then dereference 2 bytes into that to get a length
of the node, which had undefined (and quite undesired) effects.

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