bd_open(): return NULL if disc root is NULL
bd_open(): return NULL if disc root is NULL

Fixes crash when disc files are accessed later.

tourettes/libbluray,EdwardNewK/libbluray,vlc-mirror/libbluray,tourettes/libbluray,ShiftMediaProject/libbluray,Azzuro/libbluray,tourettes/libbluray,EdwardNewK/libbluray,ace20022/libbluray,koying/libbluray,koying/libbluray,koying/libbluray,ShiftMediaProject/libbluray,mwgoldsmith/bluray,vlc-mirror/libbluray,mwgoldsmith/bluray,ace20022/libbluray,mwgoldsmith/bluray,ShiftMediaProject/libbluray,koying/libbluray,Azzuro/libbluray,EdwardNewK/libbluray,vlc-mirror/libbluray,vlc-mirror/libbluray,tourettes/libbluray,ace20022/libbluray,mwgoldsmith/bluray,EdwardNewK/libbluray,Azzuro/libbluray,ShiftMediaProject/libbluray,Azzuro/libbluray,ace20022/libbluray