avcodec/vp9: add a never triggerable assert.

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.

libavcodec/vp9.c

@@ -2768,6 +2768,8 @@ static int decode_sb(AVCodecContext *ctx, int row, int col, struct VP9Filter *lf
                     }
                 }
                 break;
+            default:
+                av_assert0(0);
             }
         } else if (vp56_rac_get_prob_branchy(&s->c, p[1])) {
             bp = PARTITION_SPLIT;