avcodec/av1dec: Avoid implicit fallthrough

Fixes a -Wimplicit-fallthrough warning from Clang;
GCC does not warn about this.

Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/av1dec.c

@@ -1462,7 +1462,7 @@ static int av1_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
         case AV1_OBU_TEMPORAL_DELIMITER:
             s->raw_frame_header = NULL;
             raw_tile_group      = NULL;
-        // fall-through
+            break;
         case AV1_OBU_TILE_LIST:
         case AV1_OBU_PADDING:
             break;