irqdomain: Add missing NULL pointer check in irq_domain_create_hierarchy()

The recent switch to per-domain locking caused a NULL dereference in
irq_domain_create_hierarchy(), as Xen code is calling
msi_create_irq_domain() with a NULL parent pointer.

Fix that by testing parent to be set before dereferencing it. For a
non-existing parent the irqdomain's root will stay to point to
itself.

Fixes: 9dbb8e34 ("irqdomain: Switch to per-domain locking")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230223083800.31347-1-jgross@suse.com