avcodec/webvttdec: add some memory checks

libavcodec/webvttdec.c

@@ -74,6 +74,7 @@ static int webvtt_event_to_ass(AVBPrint *buf, const char *p)
 static int webvtt_decode_frame(AVCodecContext *avctx,
                                void *data, int *got_sub_ptr, AVPacket *avpkt)
 {
+    int ret = 0;
     AVSubtitle *sub = data;
     const char *ptr = avpkt->data;
     AVBPrint buf;
@@ -83,10 +84,12 @@ static int webvtt_decode_frame(AVCodecContext *avctx,
         int ts_start     = av_rescale_q(avpkt->pts, avctx->time_base, (AVRational){1,100});
         int ts_duration  = avpkt->duration != -1 ?
                            av_rescale_q(avpkt->duration, avctx->time_base, (AVRational){1,100}) : -1;
-        ff_ass_add_rect(sub, buf.str, ts_start, ts_duration, 0);
+        ret = ff_ass_add_rect_bprint(sub, &buf, ts_start, ts_duration, 0);
     }
-    *got_sub_ptr = sub->num_rects > 0;
     av_bprint_finalize(&buf, NULL);
+    if (ret < 0)
+        return ret;
+    *got_sub_ptr = sub->num_rects > 0;
     return avpkt->size;
 }