lavc/h264dec: make sure a slice is decoded before finishing setup

Fixes regression in fate-h264-attachment-631 with THREADS=8 introduced
by bdbbb8f11e.

libavcodec/h264dec.c

@@ -679,13 +679,13 @@ again:
             if (sl->redundant_pic_count > 0)
                 break;
 
+            if (h->current_slice == 1) {
             if (avctx->active_thread_type & FF_THREAD_FRAME && !h->avctx->hwaccel &&
                 i >= nals_needed && !h->setup_finished && h->cur_pic_ptr) {
                 ff_thread_finish_setup(avctx);
                 h->setup_finished = 1;
             }
 
-            if (h->current_slice == 1) {
                 if (h->avctx->hwaccel &&
                     (ret = h->avctx->hwaccel->start_frame(h->avctx, buf, buf_size)) < 0)
                     goto end;