perf/x86/intel/bts: Make sure debug store is valid

Since commit 4d4c474 ("perf/x86/intel/bts: Fix BTS PMI detection")
my box goes boom on boot:

| .... node  #0, CPUs:      #1 #2 #3 #4 #5 #6 #7
| BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
| IP: [<ffffffff8100c463>] intel_bts_interrupt+0x43/0x130
| Call Trace:
|  <NMI> d [<ffffffff8100b341>] intel_pmu_handle_irq+0x51/0x4b0
|  [<ffffffff81004d47>] perf_event_nmi_handler+0x27/0x40

This happens because the code introduced in this commit dereferences the
debug store pointer unconditionally. The debug store is not guaranteed to
be available, so a NULL pointer check as on other places is required.

Fixes: 4d4c474 ("perf/x86/intel/bts: Fix BTS PMI detection")
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

arch/x86/events/intel/bts.c

@@ -455,7 +455,7 @@ int intel_bts_interrupt(void)
 	 * The only surefire way of knowing if this NMI is ours is by checking
 	 * the write ptr against the PMI threshold.
 	 */
-	if (ds->bts_index >= ds->bts_interrupt_threshold)
+	if (ds && (ds->bts_index >= ds->bts_interrupt_threshold))
 		handled = 1;
 
 	/*