Since getpriority() can legitimately return the value -1, it is necessary
to clear the external variable errno prior to the call, then check
it afterward to determine if -1 is an error or a legitimate value.

Cc: Filipe Brandenburger <filbranden@google.com>
Fixes: 16e673c2f6a0 ("cr-check: Inspect errno on syscall failures")
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>