cmdlinenv00 test: fix for clang

clang complains:
> cmdlinenv00.c:35:11: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
>                 if (ret < 0) {
>                     ~~~ ^ ~

The fix is to use ssize_t not size_t.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>