lavc: set defaults in internal codec frames

This is required specifically for setting frame->format to -1,
otherwise it will be set to 0 = PIX_FMT_YUV420P and code reading
the format from the output decoded frame will get misled.

In particular fix regressions occurring with the pending vsrc_buffer
patch.

libavcodec/tiertexseqv.c

@@ -180,6 +180,7 @@ static av_cold int seqvideo_decode_init(AVCodecContext *avctx)
     seq->avctx = avctx;
     avctx->pix_fmt = PIX_FMT_PAL8;
 
+    avcodec_get_frame_defaults(&seq->frame);
     seq->frame.data[0] = NULL;
 
     return 0;