vorbisenc: Check the return value of av_frame_clone

Prevents a segfault when alloc fails.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/vorbisenc.c

@@ -1093,9 +1093,13 @@ static int vorbis_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     PutBitContext pb;
 
     if (frame) {
+        AVFrame *clone;
         if ((ret = ff_af_queue_add(&venc->afq, frame)) < 0)
             return ret;
-        ff_bufqueue_add(avctx, &venc->bufqueue, av_frame_clone(frame));
+        clone = av_frame_clone(frame);
+        if (!clone)
+            return AVERROR(ENOMEM);
+        ff_bufqueue_add(avctx, &venc->bufqueue, clone);
     } else
         if (!venc->afq.remaining_samples)
             return 0;