avcodec/h264_slice: don't copy frame data during error concealment

In addition to the fact that av_image_copy() cannot handle hardware pixel formats,
h->short_ref[0]->f may not be writable at this point.

Based on a patch by Hendrik Leppkes.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/h264_slice.c

@@ -1599,14 +1599,15 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
                 ff_thread_await_progress(&prev->tf, INT_MAX, 0);
                 if (prev->field_picture)
                     ff_thread_await_progress(&prev->tf, INT_MAX, 1);
-                av_image_copy(h->short_ref[0]->f->data,
-                              h->short_ref[0]->f->linesize,
-                              (const uint8_t **)prev->f->data,
-                              prev->f->linesize,
-                              prev->f->format,
-                              prev->f->width,
-                              prev->f->height);
+                ff_thread_release_buffer(h->avctx, &h->short_ref[0]->tf);
+                h->short_ref[0]->tf.f = h->short_ref[0]->f;
+                ret = ff_thread_ref_frame(&h->short_ref[0]->tf, &prev->tf);
+                if (ret < 0)
+                    return ret;
                 h->short_ref[0]->poc = prev->poc + 2U;
+                ff_thread_report_progress(&h->short_ref[0]->tf, INT_MAX, 0);
+                if (h->short_ref[0]->field_picture)
+                    ff_thread_report_progress(&h->short_ref[0]->tf, INT_MAX, 1);
             } else if (!h->frame_recovered && !h->avctx->hwaccel)
                 ff_color_frame(h->short_ref[0]->f, c);
             h->short_ref[0]->frame_num = h->poc.prev_frame_num;