h264_slice: Wait for refs to be available before we use them in error concealment

This could happen when there was a frame number gap and frame threading was used.

This fixes #5458.
Debugging-by: Ronald S. Bultje <rsbultje@gmail.com>
Debugging-by: Justin Ruggles <justin.ruggles@gmail.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/h264_slice.c

@@ -1424,6 +1424,9 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
                 h->short_ref[0]->f->width == prev->f->width &&
                 h->short_ref[0]->f->height == prev->f->height &&
                 h->short_ref[0]->f->format == prev->f->format) {
+                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,