ARM: perf: check that we have a platform device when reserving PMU

Attempting to use a hardware counter on a platform with a supported PMU
but where the platform_device (defining the interrupts) has not been
registered results in a NULL pointer dereference.

This patch fixes the problem by checking that we actually have a platform
device registered before attempting to grab the interrupts.
Reported-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm/kernel/perf_event.c

@@ -402,6 +402,9 @@ armpmu_reserve_hardware(struct arm_pmu *armpmu)
 	int i, err, irq, irqs;
 	struct platform_device *pmu_device = armpmu->plat_device;
 
+	if (!pmu_device)
+		return -ENODEV;
+
 	err = reserve_pmu(armpmu->type);
 	if (err) {
 		pr_warning("unable to reserve pmu\n");