ffmpeg patch - fix vaapi hardware configuration

ffmpeg.patch

--- a/libavutil/hwcontext_vaapi.c       2017-04-13 03:55:55.000000000 +0200
+++ b/libavutil/hwcontext_vaapi.c       2017-04-21 18:24:20.575277399 +0200
@@ -230,7 +230,9 @@ static int vaapi_frames_get_constraints(
             av_assert0(j == pix_fmt_count);
             constraints->valid_sw_formats[j] = AV_PIX_FMT_NONE;
         }
-    } else {
+    }
+
+    if (!config || pix_fmt_count == 0) {
         // No configuration supplied.
         // Return the full set of image formats known by the implementation.
         constraints->valid_sw_formats = av_malloc_array(ctx->nb_formats + 1,