Fix StreamStream::close() when called from shutdown hook.
Fix StreamStream::close() when called from shutdown hook.

splunk/splunk-sdk-php,splunk/splunk-sdk-php,shakeelmohamed/splunk-sdk-php,unikent/splunk-sdk-php,unikent/splunk-sdk-php,splunk/splunk-sdk-php,shakeelmohamed/splunk-sdk-php,unikent/splunk-sdk-php,shakeelmohamed/splunk-sdk-php