group/group_signal: cancel dispatch signal when the number

MIRTOS-647

of members in the group is zero.

Repetition step:
If using waitpid() to wait a task has exited, the waitpid will use
nxsing_kill with signal number 0 to determine if that task is still
alive, and will call group_signal to dispatch signal for each member
of the group and perform signal handing checks, it will assert at
sched/group/group_signal.c:261.

Change-Id: I75ef015a261101277436d742897ce68fed44b5da
Signed-off-by: Jiuzhu Dong <dongjiuzhu1@xiaomi.com>
(cherry picked from commit 73008471e4e4a5b1e3aa6cc0d25b5295aec32bb1)

sched/group/group_signal.c

@@ -245,11 +245,15 @@ int group_signal(FAR struct task_group_s *group, FAR siginfo_t *siginfo)
        * signal to a pending state.
        */
 
-      else /* if (info.dtcb) */
+      else if (info.dtcb)
         {
-          DEBUGASSERT(info.dtcb);
           tcb = info.dtcb;
         }
+      else
+        {
+          ret = -ECHILD;
+          goto errout;
+        }
 
       /* Now deliver the signal to the selected group member */