make swfdec_buffer_get_bits return NULL if no more data available
make swfdec_buffer_get_bits return NULL if no more data available

it used to assert before

freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec,mltframework/swfdec