dnxhdenc: assert ff_dnxhd_get_cid_table() return value

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dnxhdenc.c

@@ -272,6 +272,7 @@ static int dnxhd_encode_init(AVCodecContext *avctx)
     av_log(avctx, AV_LOG_DEBUG, "cid %d\n", ctx->cid);
 
     index = ff_dnxhd_get_cid_table(ctx->cid);
+    av_assert0(index >= 0);
     ctx->cid_table = &ff_dnxhd_cid_table[index];
 
     ctx->m.avctx = avctx;