Ensure invalid items are not written to FastBackend in ChainedFast

Problem/Motivation

Avoid writing invalid data to the fast backend.

Only relevant if/when someone calls getMultiple() with invalid == TRUE.

Marking issue NOVICE ONLY for creating a patch out of the below code.

Steps to reproduce

- Call getMultiple() with $allow_invalid = TRUE.
- See that fastBackend is happily setting invalid data, which will never be useful

Proposed resolution

<code>
diff --git a/web/core/lib/Drupal/Core/Cache/ChainedFastBackend.php b/web/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
--- a/web/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
+++ b/web/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
@@ -166,7 +166,10 @@ public function getMultiple(&$cids, $allow_invalid = FALSE) {
         $cache[$item->cid] = $item;
         // Don't write the cache tags to the fast backend as any cache tag
         // invalidation results in an invalidation of the whole fast backend.
-        $this->fastBackend->set($item->cid, $item->data, $item->expire);
+        if (!$allow_invalid || $item->valid) {
+          $this->fastBackend->set($item->cid, $item->data, $item->expire);
+        }
       }
     }

Remaining tasks

Review

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A