Check for IFD entries overflowing an int.

Change-Id: Ib473e2ad1d067fe041d345e21424a89e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/image/+/790223
Reviewed-by: Neal Patel <neal@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>