The ensure clause should keep its ci after its execution; fix #3406

This issue was reported by https://hackerone.com/ston3

src/vm.c

@@ -280,12 +280,14 @@ ecall(mrb_state *mrb, int i)
   mrb_callinfo *ci;
   mrb_value *self = mrb->c->stack;
   struct RObject *exc;
+  int cioff;
 
   if (i<0) return;
   p = mrb->c->ensure[i];
   if (!p) return;
   if (mrb->c->ci->eidx > i)
     mrb->c->ci->eidx = i;
+  cioff = mrb->c->ci - mrb->c->cibase;
   ci = cipush(mrb);
   ci->stackent = mrb->c->stack;
   ci->mid = ci[-1].mid;
@@ -298,6 +300,7 @@ ecall(mrb_state *mrb, int i)
   exc = mrb->exc; mrb->exc = 0;
   mrb_run(mrb, p, *self);
   mrb->c->ensure[i] = NULL;
+  mrb->c->ci = mrb->c->cibase + cioff;
   if (!mrb->exc) mrb->exc = exc;
 }