Close ask_dialog and delete it in after exec()

Fixed Moligaloo/QSanguosha#197.

This patch makes sure that `ask_dialog` is freed every time **after** exec(), so that event loop during `ask_dialog->exec()` would not observe freed `ask_dialog` which causes crash.

Two occurrences of `delete ask_dialog` in `Client::askForSuit` and `Client::askForKingdom` should be audited as well.