vm: execute ensure at the top of the fiber; fix #2903

src/vm.c

@@ -1497,6 +1497,9 @@ RETRY_TRY_BLOCK:
             MRB_THROW(prev_jmp);
           }
           if (ci == mrb->c->cibase) {
+            while (eidx > 0) {
+              ecall(mrb, --eidx);
+            }
             if (ci->ridx == 0) {
               if (mrb->c == mrb->root_c) {
                 regs = mrb->c->stack = mrb->c->stbase;