ibus-chewing should check errors returned by libchewing

[kanru]

測試 libchewing-0.7.0-rc.1 的時候發現的

如果 chewing_new 回傳 null 的話， ibus-chewing 會進入一個無窮迴圈

Found while testing libchewing-0.7.0-rc.1. If chewing_new returns null then ibus-chewing will enter a infinite loop.

We should check return values and handle them.