perf/x86/intel: Fix constraint access

Sasha reported that we can get here with .idx==-1, and
cpuc->event_constraints unallocated.

Suggested-by: Stephane Eranian <eranian@google.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter 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: Ingo Molnar <mingo@kernel.org>