fix the code for testing message integrity

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
fix a regression introduced in a01f97eae41bcd759bfc521d84053552cf38d521

Internal buffers must be aligned on 8 bytes because a lot of framework
code assumes that this alignment is there.

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>