smp: assert that all affected CPUs are online in on_selected_cpus()

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

--- a/xen/common/smp.c
+++ b/xen/common/smp.c
@@ -53,6 +53,7 @@ void on_selected_cpus(
     unsigned int nr_cpus;
 
     ASSERT(local_irq_is_enabled());
+    ASSERT(cpumask_subset(selected, &cpu_online_map));
 
     spin_lock(&call_lock);