Fix invalid use of NULL in epoll_pwait2(2) test

epoll_pwait2(2)'s second argument should be nonnull.  We're going to add
__nonnull to the prototype, so let's fix the test accordingly.  We can
use a dummy variable to avoid passing NULL.

Reported-by: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>