perf/core: Fix reversed NULL check in perf_event_groups_less()

This NULL check is reversed so it leads to a Smatch warning and
presumably a NULL dereference.

    kernel/events/core.c:1598 perf_event_groups_less()
    error: we previously assumed 'right->cgrp->css.cgroup' could be null
	(see line 1590)

Fixes: 95ed6c70 ("perf/cgroup: Order events in RB tree by cgroup id")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200312105637.GA8960@mwanda

kernel/events/core.c

@@ -1586,7 +1586,7 @@ perf_event_groups_less(struct perf_event *left, struct perf_event *right)
 			 */
 			return true;
 		}
-		if (!right->cgrp || right->cgrp->css.cgroup) {
+		if (!right->cgrp || !right->cgrp->css.cgroup) {
 			/*
 			 * Right has no cgroup but left does, no cgroups come
 			 * first.