Check init_get_bits' result for NULL, to avoid dereferencing a NULL
pointer later (CWE-476).
Without this, a segfault happens when trying to decode a handcrafted
ogg-flac file with an absurdly long (e.g. 268435455 bytes) ogg header.
Co-authored-by: James Almer <jamrial@gmail.com>
Signed-off-by: Paul Arzelier <paul.arzelier@free.fr>