As described on the mailing-list, messages that are in paused state won't be removed from the queue when soup_session_abort() is called. As a consequence the messages are leaked and soup_message_queue_destroy() hits an assertion:

 soup_message_queue_destroy: assertion `queue->head == NULL' failed

This appears to be caused by commit 5cb220971a, which was done to fix bug #651146. This commit introduced code in process_queue_item() that skips paused messages. Due to this code paused messages that are cancelled won't be removed from the queue.