xen/arm: vgic-v3-its: Make vgic_v3_its_free_domain idempotent

xen/arm: vgic-v3-its: Make vgic_v3_its_free_domain idempotent

vgic_v3_its_free_domain may be called before vgic_v3_its_init_domain if
the vGIC was failing to initalize itself. This means the list would be
unitialized and result in a crash.

Thankfully, we only allow ITS for the hardware domain. So the crash is
not a security issue. Fix it by checking whether the list the NULL.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>