ssltestlib: Tell compiler we don't care about the value when we don't

In mempacket_test_read(), we've already fetched the top value of the
stack, so when we shift the stack, we don't care for the value.  The
compiler needs to be told, or it will complain harshly when we tell it
to be picky.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 1c288878)