Fix instance reuse bug in out-in transition #1874 (#2127)

**What kind of change does this PR introduce?** (check at least one)

- [x] Bugfix
- [ ] Feature
- [ ] Code style update
- [ ] Refactor
- [ ] Build-related changes
- [ ] Other, please describe:

**Does this PR introduce a breaking change?** (check one)

- [ ] Yes
- [x] No

If yes, please describe the impact and migration path for existing applications:

**The PR fulfills these requirements:**

- [x] It's submitted to the `dev` branch for v2.x (or to a previous version branch), _not_ the `master` branch
- [x] When resolving a specific issue, it's referenced in the PR's title (e.g. `fix #xxx[,#xxx]`, where "xxx" is the issue number)
- [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
- [ ] New/updated tests are included

If adding a **new feature**, the PR's description includes:
- [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

**Other information:**

This is a PR for #1874 , cc: @LinusBorg

src/history/base.js

@@ -318,7 +318,10 @@ function poll (
   key: string,
   isValid: () => boolean
 ) {
-  if (instances[key]) {
+  if (
+    instances[key] &&
+    !instances[key]._isBeingDestroyed // do not reuse being destroyed instance
+  ) {
     cb(instances[key])
   } else if (isValid()) {
     setTimeout(() => {