Don't update the "image size" when adding pe certificates

The image size field relates to the size of the mapped image, and as
such should not include change for certificate size.

This should resolve https://github.com/vathpela/pesign/issues/1

Signed-off-by: Peter Jones <pjones@redhat.com>
Check for failure on result of pe_alloccert()

Signed-off-by: Peter Jones <pjones@redhat.com>
Validate signature number input better, and update the file appropriately

We weren't checking the bounds of the signature number correctly, and
then were also failing to update the structure during the remove
operation.

Signed-off-by: Peter Jones <pjones@redhat.com>