h264: dont mess with chroma planes for grayscale h264.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/svq3.c

@@ -823,6 +823,7 @@ static av_cold int svq3_decode_init(AVCodecContext *avctx)
     s->flags2 = avctx->flags2;
     s->unrestricted_mv = 1;
     h->is_complex=1;
+    h->sps.chroma_format_idc = 1;
     avctx->pix_fmt = avctx->codec->pix_fmts[0];
 
     if (!s->context_initialized) {