score: Avoid _Scheduler_Get_by_CPU_index( 0 )

Avoid use of processor index 0 which may have no scheduler assigned.

cpukit/score/include/rtems/score/coremuteximpl.h

@@ -237,7 +237,7 @@ _CORE_ceiling_mutex_Get_scheduler(
 #if defined(RTEMS_SMP)
   return the_mutex->scheduler;
 #else
-  return _Scheduler_Get_by_CPU_index( 0 );
+  return &_Scheduler_Table[ 0 ];
 #endif
 }