Fixes PHP-680: Always free tagset in dtor function

See: https://jira.mongodb.org/browse/PHP-680?focusedCommentId=253631&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-253631

Previously, tagset would not be freed if it contained no tags. Since it is always allocated, we should free it regardless of whether tags were added to it.