coverity: Dereferencing after a null check (FORWARD_NULL)

Add proper null pointer validation and fix potential dereference after null
check in scheduler critical sections. This addresses Coverity FORWARD_NULL
defect by ensuring pointers are checked before use and maintaining defensive
coding practices throughout the scheduler implementation.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

sched/sched/sched_mergepending.c

@@ -96,7 +96,7 @@ bool nxsched_merge_pending(void)
            */
 
           for (;
-               (rtcb && ptcb->sched_priority <= rtcb->sched_priority);
+               (ptcb->sched_priority <= rtcb->sched_priority);
                rtcb = rtcb->flink)
             {
             }