Wrong start and end values would make dead cycle. Exchange the values when start value is bigger than end value.

system/dumpstack/dumpstack.c

@@ -44,6 +44,7 @@ int main(int argc, FAR char *argv[])
 {
   int spid = -1;
   int epid = -1;
+  int tmp = -1;
 
   if (argc == 2)
     {
@@ -60,6 +61,13 @@ int main(int argc, FAR char *argv[])
       spid = epid = gettid();
     }
 
+  if (spid > epid)
+    {
+      tmp = spid;
+      spid = epid;
+      epid = tmp;
+    }
+
   do
     {
       sched_dumpstack(spid);