cache tests: don't call clean on a non existing instance on tearDown

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@21222 44c647ce-9c0f-0410-b52a-842ac1e357ba

tests/Zend/Cache/CommonBackendTest.php

@@ -98,7 +98,9 @@ abstract class Zend_Cache_CommonBackendTest extends PHPUnit_Framework_TestCase {
 
     public function tearDown()
     {
-        $this->_instance->clean();
+        if ($this->_instance) {
+            $this->_instance->clean();
+        }
         $this->rmdir();
     }