h264: wait for missing slices only on frames

The EC code does not support fields currently thus it makes no
sense to wait for these cases (which also the check doesnt handle
correctly)

Fixes Ticket 2454
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -4248,7 +4248,7 @@ static int decode_slice(struct AVCodecContext *avctx, void *arg)
                     avctx->codec_id != AV_CODEC_ID_H264 ||
                     (CONFIG_GRAY && (h->flags & CODEC_FLAG_GRAY));
 
-    if (!(h->avctx->active_thread_type & FF_THREAD_SLICE)) {
+    if (!(h->avctx->active_thread_type & FF_THREAD_SLICE) && h->picture_structure == PICT_FRAME) {
         const int start_i  = av_clip(h->resync_mb_x + h->resync_mb_y * h->mb_width, 0, h->mb_num - 1);
         if (start_i) {
             int prev_status = h->er.error_status_table[h->er.mb_index2xy[start_i - 1]];