libfdisk: (gpt) fix check for beginning of protective partition

libfdisk: (gpt) fix check for beginning of protective partition

Search for the protective partition before attempting to validate that
it covers the GPT header. Some hybrid MBRs list partitions out of order.
For example, CoreOS images ship with a hybrid MBR that lists the boot
partition first and the protective partition second in order to maintain
compatibility with old versions of Xen's PV-GRUB.

Signed-off-by: Michael Marineau <michael.marineau@coreos.com>