vp8_rac_get_tree() is called with a tree of size 3, so the returned
value can not be outside [0;3]. All of the [0;3] cases are handled in
the switch, so the assert should not be triggerable by any means. A
similar change was introduced in 97962b27 / 72ca830f, with the
av_log()+AVERROR_INVALIDDATA form, suggesting it could be triggerable
somehow. This assert might help static analyzer, or simply the reader.