util: Fix possible NULL dereference

Commit 1a80b97d, which added the virCgroupHasEmptyTasks() function
forgot that the parameter @cgroup may be NULL and did not check that.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/util/vircgroup.c

@@ -3932,6 +3932,9 @@ virCgroupHasEmptyTasks(virCgroupPtr cgroup, int controller)
     int ret = -1;
     char *content = NULL;
 
+    if (!cgroup)
+        return -1;
+
     ret = virCgroupGetValueStr(cgroup, controller, "tasks", &content);
 
     if (ret == 0 && content[0] == '\0')