avutil/frame: access avframe fields directly in get_frame_defaults()

The accessors are needed only from outside libavutil.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/frame.c

@@ -101,10 +101,10 @@ static void get_frame_defaults(AVFrame *frame)
     frame->pts                   =
     frame->pkt_dts               =
     frame->pkt_pts               = AV_NOPTS_VALUE;
-    av_frame_set_best_effort_timestamp(frame, AV_NOPTS_VALUE);
-    av_frame_set_pkt_duration         (frame, 0);
-    av_frame_set_pkt_pos              (frame, -1);
-    av_frame_set_pkt_size             (frame, -1);
+    frame->best_effort_timestamp = AV_NOPTS_VALUE;
+    frame->pkt_duration        = 0;
+    frame->pkt_pos             = -1;
+    frame->pkt_size            = -1;
     frame->key_frame           = 1;
     frame->sample_aspect_ratio = (AVRational){ 0, 1 };
     frame->format              = -1; /* unknown */