don't print garbage entries.

src/vm.c

@@ -169,6 +169,9 @@ cipush(mrb_state *mrb)
   mrb->ci->eidx = eidx;
   mrb->ci->ridx = ridx;
   mrb->ci->env = 0;
+  if (mrb->ci + 1 != mrb->ciend) {
+    mrb->ci[1].mid = 0;
+  }
   return mrb->ci;
 }
 

tools/mruby/mruby.c

@@ -170,9 +170,9 @@ showcallinfo(mrb_state *mrb)
   int i;
 
   printf("trace:\n");
-  for (i = 0; &mrb->cibase[i] < mrb->ciend; i++) {
+  for (i = 1; &mrb->cibase[i] < mrb->ciend; i++) {
     ci = &mrb->cibase[i];
-    if (ci->target_class == NULL)
+    if (ci->mid == 0)
       break;
 
     if (MRB_PROC_CFUNC_P(ci->proc))