avcodec/h264dec: set AVFrame decode_error_flags in case of decoding error

set AVFrame decode_error_flags in case h->slice_ctx->er.error_occurred is set
after the call to ff_h264_execute_decode_slices. This allows the user to detect
concealed decoding errors in the call to avcodec_receive_frame
Signed-off-by: Amir Pauker <amir@livelyvideo.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264dec.c

@@ -761,6 +761,11 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)
     if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
         goto end;
 
+    // set decode_error_flags to allow users to detect concealed decoding errors
+    if ((ret < 0 || h->slice_ctx->er.error_occurred) && h->cur_pic_ptr) {
+        h->cur_pic_ptr->f->decode_error_flags |= FF_DECODE_ERROR_DECODE_SLICES;
+    }
+
     ret = 0;
 end: