prevent out-of-bounds ensure clause access; fix #2910

src/vm.c

@@ -266,6 +266,7 @@ ecall(mrb_state *mrb, int i)
   mrb_value *self = mrb->c->stack;
   struct RObject *exc;
 
+  if (i<0) return;
   p = mrb->c->ensure[i];
   if (!p) return;
   if (mrb->c->ci->eidx > i)