mod_ssl: follow up to r1036492: fix rollback in char_buffer_consume().

This needs the same bucket insertion code as in char_buffer_write(), so define
a new char_buffer_insert() helper.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1844781 13f79535-47bb-0310-9956-ffa450edef68