genirq/msi: Check for the presence of an irq domain when validating msi_ctrl

For architectures such as s390 and powerpc that do not use
irq domains for MSIs, dev->msi.domain is always NULL, so
the per-device, per-bus MSI domain is also guaranteed to
be NULL.

So checking one without checking the other is bound to result
in a splat, followed by a memory leak as we don't free the MSI
descriptors.

Add the missing check.

Reported-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/e570e70d-19bc-101b-0481-ff9a3cab3504@linux.ibm.com