task: delete the queue after deleting all of its children

This was resulting in an assertion failure later on if a queue was
being rescued from a deleted task with only one post-exec queue.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/task.cc

@@ -259,6 +259,7 @@ namespace crucible {
 		for (auto &i : tq) {
 			i->clear();
 		}
+		tq.clear();
 	}
 
 	void