x86/irq: Do not dereference irq descriptor before checking it

Having the IS_NULL_OR_ERR() check after dereferencing the pointer is
not really working well.

Move the dereference after the check.

Fixes: a782a7e 'x86/irq: Store irq descriptor in vector array'
Reported-and-tested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/irq_32.c

@@ -150,14 +150,15 @@ void do_softirq_own_stack(void)
 
 bool handle_irq(struct irq_desc *desc, struct pt_regs *regs)
 {
-	unsigned int irq = irq_desc_get_irq(desc);
+	unsigned int irq;
 	int overflow;
 
 	overflow = check_stack_overflow();
 
 	if (IS_ERR_OR_NULL(desc))
 		return false;
 
+	irq = irq_desc_get_irq(desc);
 	if (user_mode(regs) || !execute_on_irq_stack(overflow, desc, irq)) {
 		if (unlikely(overflow))
 			print_stack_overflow();