cr-dump: getpriority() can return a negative value

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>

criu/cr-dump.c

@@ -168,9 +168,10 @@ static int dump_sched_info(int pid, ThreadCoreEntry *tc)
 	 * in kernel. Thus we have to take it with us in the image.
 	 */
 
+	errno = 0;
 	ret = getpriority(PRIO_PROCESS, pid);
-	if (ret < 0) {
-		pr_perror("Can't get nice for %d", pid);
+	if (ret == -1 && errno) {
+		pr_perror("Can't get nice for %d ret %d", pid, ret);
 		return -1;
 	}