ARM: Handle no IPI being registered in show_ipi_list()

As SMP-on-UP is a valid configuration on 32bit ARM, do not assume that
IPIs are populated in show_ipi_list().
Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: kernelci.org bot <bot@kernelci.org>
Tested-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm/kernel/smp.c

@@ -541,7 +541,12 @@ void show_ipi_list(struct seq_file *p, int prec)
 	unsigned int cpu, i;
 
 	for (i = 0; i < NR_IPI; i++) {
-		unsigned int irq = irq_desc_get_irq(ipi_desc[i]);
+		unsigned int irq;
+
+		if (!ipi_desc[i])
+			continue;
+
+		irq = irq_desc_get_irq(ipi_desc[i]);
 		seq_printf(p, "%*s%u: ", prec - 1, "IPI", i);
 
 		for_each_online_cpu(cpu)