must not fail if MemCache class isn't available

[drzraf]

It can occur during a lot of situation:

• enabled in FPM but not in CLI (so wp-cli fails): I guess it's shouldn't even load if PHP_SAPI === 'cli'
• during a PHP socket switch / version upgrade
• during a debugging attempt that imply deactivating so PHP modules
• ...

In all these and others cases, the extensions should definitely NOT trigger a fatal error.