[2.0][DDC-402] Fixed issue with MemcacheCache througing E_WARNINGs if no cache entry is found.

lib/Doctrine/Common/Cache/MemcacheCache.php

@@ -71,8 +71,11 @@ class MemcacheCache extends AbstractCache
         $allSlabs = $this->_memcache->getExtendedStats('slabs');
 
         foreach ($allSlabs as $server => $slabs) {
+            if (is_array($slabs)) {
                 foreach (array_keys($slabs) as $slabId) {
                     $dump = $this->_memcache->getExtendedStats('cachedump', (int) $slabId);
+
+                    if ($dump) {
                         foreach ($dump as $entries) {
                             if ($entries) {
                                 $keys = array_merge($keys, array_keys($entries));
@@ -80,6 +83,8 @@ class MemcacheCache extends AbstractCache
                         }
                     }
                 }
+            }
+        }
         return $keys;
     }