[PATCH 1/2][4.15?] sched: fix build when NR_CPUS == 1

[Jan Beulich]

In this case the compiler is recognizing that no valid array indexes
remain, and hence e.g. reports:

core.c: In function 'cpu_schedule_up':
core.c:2769:19: error: array subscript 1 is above array bounds
of 'struct vcpu *[1]' [-Werror=array-bounds]
 2769 |     if ( idle_vcpu[cpu] == NULL )
      |          ~~~~~~~~~^~~~~

Reported-by: Connor Davis <connojda...@gmail.com>
Signed-off-by: Jan Beulich <jbeul...@suse.com>

--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -2768,6 +2768,12 @@ static int cpu_schedule_up(unsigned int
     if ( cpu == 0 )
         return 0;
 
+    /*
+     * Guard in particular against the compiler suspecting out-of-bounds
+     * array accesses below when NR_CPUS=1.
+     */
+    BUG_ON(cpu >= NR_CPUS);
+
     if ( idle_vcpu[cpu] == NULL )
         vcpu_create(idle_vcpu[0]->domain, cpu);
     else