perf/x86/intel: Fix constraint access

Sasha reported that we can get here with .idx==-1, and
cpuc->event_constraints unallocated.
Suggested-by: NStephane Eranian <eranian@google.com>
Reported-by: NSasha Levin <sasha.levin@oracle.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Fixes: b371b594 ("perf/x86: Fix event/group validation")
Signed-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/perf_event_intel.c

@@ -2316,9 +2316,12 @@ static struct event_constraint *
 intel_get_event_constraints(struct cpu_hw_events *cpuc, int idx,
 			    struct perf_event *event)
 {
-	struct event_constraint *c1 = cpuc->event_constraint[idx];
+	struct event_constraint *c1 = NULL;
 	struct event_constraint *c2;
 
+	if (idx >= 0) /* fake does < 0 */
+		c1 = cpuc->event_constraint[idx];
+
 	/*
 	 * first time only
 	 * - static constraint: no change across incremental scheduling calls