avcodec/dnxhddec: reset cur_field for non interlaced frames

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

libavcodec/dnxhddec.c

@@ -136,6 +136,8 @@ static int dnxhd_decode_header(DNXHDContext *ctx, AVFrame *frame,
         frame->top_field_first  = first_field ^ ctx->cur_field;
         av_log(ctx->avctx, AV_LOG_DEBUG,
                "interlaced %d, cur field %d\n", buf[5] & 3, ctx->cur_field);
+    } else {
+        ctx->cur_field = 0;
     }
 
     ctx->height = AV_RB16(buf + 0x18);