perf/core: Fix potential NULL deref

jira VULN-6760
cve-bf CVE-2023-5717
commit-author Peter Zijlstra <peterz@infradead.org>
commit a71ef31

Smatch is awesome.

Fixes: 32671e3 ("perf: Disallow mis-matched inherited group reads")
	Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
	Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
	Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit a71ef31)
	Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98

kernel/events/core.c

@@ -13143,7 +13143,8 @@ static int inherit_group(struct perf_event *parent_event,
 		    !perf_get_aux_event(child_ctr, leader))
 			return -EINVAL;
 	}
-	leader->group_generation = parent_event->group_generation;
+	if (leader)
+		leader->group_generation = parent_event->group_generation;
 	return 0;
 }