arm/vgic-v3: Fix data abort in get_vcpu_from_rdist()

Just because vcpu_id is within d->max_vcpus doesn't mean d->vcpu[vcpu_id] is
non-NULL.  Use the proper accessor to look up a vcpu object.

Reported-by: Michal Orzel <michal.orzel@amd.com>
Fixes: acf65e5a25 ("xen/arm: vgic-v3: Emulate correctly the re-distributor")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien@xen.org>

xen/arch/arm/vgic-v3.c

@@ -1111,10 +1111,10 @@ static struct vcpu *get_vcpu_from_rdist(struct domain *d,
     unsigned int vcpu_id;
 
     vcpu_id = region->first_cpu + ((gpa - region->base) / GICV3_GICR_SIZE);
-    if ( unlikely(vcpu_id >= d->max_vcpus) )
-        return NULL;
 
-    v = d->vcpu[vcpu_id];
+    v = domain_vcpu(d, vcpu_id);
+    if ( !v )
+        return NULL;
 
     *offset = gpa - v->arch.vgic.rdist_base;