Fix crash in player example when close the window while playing

When it is playing and close the window:
1. The player object will be destroyed first (all QString's).
2. Next parent QWidget will be destroyed with all children.
3. While destroying QMediaPlayer from Player object, the playback will be stopped.
4. The status will be changed and Player::statusChanged will be called.
   The connection still exists.
   And since all string members are already freed, it produces a crash.

Change-Id: I2c2fdca97f0ff4b300527b550ecc0c12b04e5b4e
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>