sched/backtrace: simplify the assign of tcb in case of negative pid

Signed-off-by: chao.an <anchao@xiaomi.com>

sched/sched/sched_backtrace.c

@@ -42,21 +42,16 @@
 
 int sched_backtrace(pid_t tid, FAR void **buffer, int size)
 {
-  FAR struct tcb_s *rtcb;
+  FAR struct tcb_s *rtcb = NULL;
 
-  if (tid < 0)
-    {
-      rtcb = running_task();
-    }
-  else
+  if (tid >= 0)
     {
       rtcb = nxsched_get_tcb(tid);
-    }
-
       if (rtcb == NULL)
         {
           return 0;
         }
+    }
 
   return up_backtrace(rtcb, buffer, size);
 }