Should clear method name

- Fix method name in top-level
- Fix SEGV when call Exception#backtrace if callinfo over CALLINFO_INIT_SIZE(32)

src/vm.c

@@ -2434,6 +2434,7 @@ mrb_top_run(mrb_state *mrb, struct RProc *proc, mrb_value self, unsigned int sta
     return mrb_vm_run(mrb, proc, self, stack_keep);
   }
   ci = cipush(mrb);
+  ci->mid = 0;
   ci->nregs = 1;   /* protect the receiver */
   ci->acc = CI_ACC_SKIP;
   ci->target_class = mrb->object_class;