perf/core: Fix potential NULL deref

jira VULN-7623
cve-bf CVE-2023-5717
commit-author Peter Zijlstra <[email protected]>
commit a71ef31

Smatch is awesome.

Fixes: 32671e3 ("perf: Disallow mis-matched inherited group reads")
	Reported-by: Dan Carpenter <[email protected]>
	Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
	Signed-off-by: Ingo Molnar <[email protected]>
(cherry picked from commit a71ef31)
	Signed-off-by: Marcin Wcisło <[email protected]>

Author: pvts-mat

kernel/events/core.c

@@ -11031,7 +11031,8 @@ static int inherit_group(struct perf_event *parent_event,
 		if (IS_ERR(child_ctr))
 			return PTR_ERR(child_ctr);
 	}
-	leader->group_generation = parent_event->group_generation;
+	if (leader)
+		leader->group_generation = parent_event->group_generation;
 	return 0;
 }