x86 / perf: Fix suspend to RAM on HP nx6325

Commit 3f6da390
(perf: Rework and fix the arch CPU-hotplug hooks) broke suspend to
RAM on my HP nx6325 (and most likely on other AMD-based boxes too)
by allowing amd_pmu_cpu_offline() to be executed for CPUs that are
going offline as part of the suspend process.  The problem is that
cpuhw->amd_nb may be NULL already, so the function should make sure
it's not NULL before accessing the object pointed to by it.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/cpu/perf_event_amd.c

@@ -348,10 +348,12 @@ static void amd_pmu_cpu_offline(int cpu)
 
 	raw_spin_lock(&amd_nb_lock);
 
+	if (cpuhw->amd_nb) {
 		if (--cpuhw->amd_nb->refcnt == 0)
 			kfree(cpuhw->amd_nb);
 
 		cpuhw->amd_nb = NULL;
+	}
 
 	raw_spin_unlock(&amd_nb_lock);
 }