[nbi] Avoid harmless integer overflows in image length checks

[nbi] Avoid harmless integer overflows in image length checks

Fix the checks against reading beyond the image length when executing
an NBI image.

This change has absolutely no security impact: an NBI image will
obtain control of the system in ring 0 anyway, and so a "malicious"
NBI image with malformed length fields cannot do anything that it
would not already be able to do simply by being executed.  However,
fixing these harmless integer overflows costs very little and reduces
unwanted noise from security reviewers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>