fix: (@vue/apollo-option) memory leak in wrapped ssrRender

Fixes vuejs#1550

Two leaks were fixed:

1) Prevents repeatedly wrapping `ssrRender` by checking if it's already
   been wrapped. Added a `__IS_VUE_APOLLO_WRAPPED` boolean to track this.
   I verified that this was actually happening by throwing an error if
   it was already wrapped, and I observed the error.

2) `this.$options.ssrRender` doesn't always exist, but this.$apollo
   does. When the new wrapped `ssrRender` was called, it would throw,
   which prevented the `destroy.call(this)` line from running. The fix
   here was to not create a wrapped `ssrRender` if there isn't an original
   one.

packages/vue-apollo-option/src/mixin.js

@@ -115,11 +115,14 @@ export function installMixin (app, provider) {
       if (isServer) {
         // Patch render function to cleanup apollo
         const render = this.$options.ssrRender
+        if (!render) return
+        if (render.__IS_VUE_APOLLO_WRAPPED) return
         this.$options.ssrRender = (h) => {
           const result = render.call(this, h)
           destroy.call(this)
           return result
         }
+        this.$options.ssrRender.__IS_VUE_APOLLO_WRAPPED = true
       }
     },