cgptlib: Always zero-initialize GPT entries buffers

ClusterFuzz still managed to find cases where we try to CRC a GPT
entries buffer that wasn't initialized. Not that that's really an issue
or anything... but this patch should shut it up.

BRANCH=none
BUG=chromium:1155876
TEST=none

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I3d0b4f34693d87b66513f398dd13441aba543c3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2580110
Reviewed-by: Joel Kitching <kitching@chromium.org>