irqdomain: Prevent potential NULL pointer dereference in irq_domain_push_irq()

This code generates a Smatch warning:

kernel/irq/irqdomain.c:1511 irq_domain_push_irq()
warn: variable dereferenced before check 'root_irq_data' (see line 1508)

irq_get_irq_data() can return a NULL pointer, but the code dereferences
the returned pointer before checking it.

Move the NULL pointer check before the dereference.

[ tglx: Rewrote changelog to be precise and conforming to the instructions
in submitting-patches and added a Fixes tag. Sigh! ]

Fixes: 495c38d3001f ("irqdomain: Add irq_domain_{push,pop}_irq() functions")
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: David Daney <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/20170825121409.6rfv4vt6ztz2oqkt@mwanda

kernel/irq/irqdomain.c

···
 	if (WARN_ON(!irq_domain_is_hierarchy(domain)))
 		return -EINVAL;
 
-	if (domain->parent != root_irq_data->domain)
+	if (!root_irq_data)
 		return -EINVAL;
 
-	if (!root_irq_data)
+	if (domain->parent != root_irq_data->domain)
 		return -EINVAL;
 
 	child_irq_data = kzalloc_node(sizeof(*child_irq_data), GFP_KERNEL,