avcodec/mpeg12dec: do not trust AVCodecContext input dimensions

Fixes initial wtv dimensions
Fixes Ticket4070
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12dec.c

@@ -1120,6 +1120,10 @@ static av_cold int mpeg_decode_init(AVCodecContext *avctx)
     MpegEncContext *s2 = &s->mpeg_enc_ctx;
 
     ff_mpv_decode_defaults(s2);
+
+    if (   avctx->codec_tag != AV_RL32("VCR2")
+        && avctx->codec_tag != AV_RL32("BW10"))
+        avctx->coded_width = avctx->coded_height = 0; // do not trust dimensions from input
     ff_mpv_decode_init(s2, avctx);
 
     s->mpeg_enc_ctx.avctx  = avctx;