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: 495c38d3 ("irqdomain: Add irq_domain_{push,pop}_irq() functions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: David Daney <david.daney@cavium.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kernel-janitors@vger.kernel.org
Link: http://lkml.kernel.org/r/20170825121409.6rfv4vt6ztz2oqkt@mwanda