Update MemcacheEngine.php

removed 0 from getExtendedStats('slabs') in my last commit, someone point that out in the comment. Adding it back.
Merge pull request #2861 from ujwaltrivedi/master

Get unlimited number of slabs when clearing cache keys in memcache.
Fix Validation::decimal() not working with localized floats.

Use similar workarounds as DboSource::value() for accepting localized
floats.

Fixes #2853