vaapi_decode: Clear parameter buffers to fix picture reuse

When decoding interlaced pictures, the structure is reused to render
to the same surface twice.  The parameter buffers were not being
cleared, which caused the i965 driver to error out.

libavcodec/vaapi_decode.c

@@ -201,6 +201,7 @@ int ff_vaapi_decode_issue(AVCodecContext *avctx,
         AV_VAAPI_DRIVER_QUIRK_RENDER_PARAM_BUFFERS)
         ff_vaapi_decode_destroy_buffers(avctx, pic);
 
+    pic->nb_param_buffers = 0;
     pic->nb_slices        = 0;
     pic->slices_allocated = 0;
     av_freep(&pic->slice_buffers);