KVM: arm/arm64: Fix KVM_VGIC_V3_ADDR_TYPE_REDIST read

When reading the base address of the a REDIST region
through KVM_VGIC_V3_ADDR_TYPE_REDIST we expect the
redistributor region list to be populated with a single
element.

However list_first_entry() expects the list to be non empty.
Instead we should use list_first_entry_or_null which effectively
returns NULL if the list is empty.

Fixes: dbd9733a

 ("KVM: arm/arm64: Replace the single rdist region by a list")
Cc: <Stable@vger.kernel.org> # v4.18+
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210412150034.29185-1-eric.auger@redhat.com