bug #781 Fix #426: Prevent fatal error during unserialization (PatchR…

…anger)

This PR was merged into the 5.x branch.

Discussion
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Fix #426: Prevent fatal error during unserialization

## Description

Refer to #426 .
## Solution

I've looked for all of `__destruct()` implementations regarding cache usage. That was the only one - so it should completely fix the aforementioned issue.

Commits
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fb0379a Fix #426: Prevent fatal error during unserialization

lib/classes/Swift/Mime/SimpleMimeEntity.php

@@ -807,7 +807,9 @@ private function _childSortAlgorithm($a, $b)
      */
     public function __destruct()
     {
-        $this->_cache->clearAll($this->_cacheKey);
+        if ($this->_cache instanceof Swift_KeyCache) {
+            $this->_cache->clearAll($this->_cacheKey);
+        }
     }
 
     /**