If the PHP memory limit is hit whilst in the SDK then lcb can be left in an invalid state

If the PHP memory limit is hit whilst in the SDK then the libcouchbase object in use can be left in an invalid state.

Recreation, use Nginx + PHP-FPM and run (May need to refresh a few times to use up all of FPM's child processes):

```
<?php
$cluster = new CouchbaseCluster('couchbase://192.168.56.1');
$bucket = $cluster->openBucket('default');

// Creates a 30+mb string, set larger if needed
$doc = str_repeat("0", pow(2,25));
$bucket->upsert('hello', $doc);
var_dump($bucket->get('hello'));
```

This will effectively break all future connections to that bucket for that process as the lcb_t object is broken. In this situation the SDK starts to return null.

I suspect this is from emalloc failing in the SDK function but PHP not ending the request until the SDK returns. Since lcb is malloc'd itself it can retain this invalid state between requests.

Possible mitigation here, but it would be nice to check if emalloc succeeded as well:
https://github.com/couchbaselabs/php-couchbase/pull/9