genirq/msi: Prevent NULL pointer dereference in msi_domain_debug_show()

irq_domain_debug_show_one() calls msi_domain_debug_show() with a non-NULL
domain pointer and a NULL irq_data pointer. irq_debug_show_data() calls it
with a NULL domain pointer.

The domain pointer is not used, but the irq_data pointer is required to be
non-NULL and lacks a NULL pointer check.

Add the missing NULL pointer check to ensure there is a non-NULL irq_data
pointer in msi_domain_debug_show() before dereferencing it.

[ tglx: Massaged change log ]

Fixes: 01499ae6 ("genirq/msi: Expose MSI message data in debugfs")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250430124836.49964-2-ajones@ventanamicro.com