vaapi_h264: Fix VUI max_dec_frame_buffering

This should refer to the existing SPS structure, not the VAAPI sequence
parameter buffer (which is not yet initialised).

From ffmpeg commit f31478ba.
Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavcodec/vaapi_encode_h264.c

@@ -453,7 +453,7 @@ static int vaapi_encode_h264_init_sequence_params(AVCodecContext *avctx)
     sps->vui.log2_max_mv_length_horizontal = 16;
     sps->vui.log2_max_mv_length_vertical   = 16;
     sps->vui.max_num_reorder_frames        = (ctx->b_per_p > 0);
-    sps->vui.max_dec_frame_buffering       = vseq->max_num_ref_frames;
+    sps->vui.max_dec_frame_buffering       = sps->max_num_ref_frames;
 
     pps->nal_unit_header.nal_ref_idc = 3;
     pps->nal_unit_header.nal_unit_type = H264_NAL_PPS;