h264: only allow ending a field/starting a new one before finish_setup()

Doing this after ff_thread_finish_setup() is called is invalid and can
conflict with reads from the other thread.

libavcodec/h264_slice.c

@@ -1381,6 +1381,7 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl,
     if (ret < 0)
         return ret;
 
+    if (!h->setup_finished) {
         if (sl->first_mb_addr == 0) { // FIXME better field boundary detection
             if (h->current_slice && h->cur_pic_ptr && FIELD_PICTURE(h)) {
                 ff_h264_field_end(h, sl, 1);
@@ -1400,7 +1401,10 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl,
             ret = h264_field_start(h, sl, nal);
             if (ret < 0)
                 return ret;
-    } else {
+        }
+    }
+
+    if (h->current_slice > 0) {
         if (h->ps.pps != (const PPS*)h->ps.pps_list[sl->pps_id]->data) {
             av_log(h->avctx, AV_LOG_ERROR, "PPS changed between slices\n");
             return AVERROR_INVALIDDATA;