libtwolame: prevent a NULL pointer dereference

CC: libav-stable@libav.org
Bug-Id: CID 1250330 / CID 1250335

libavcodec/libtwolame.c

@@ -148,8 +148,8 @@ static int twolame_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     if (ret < 0)  // twolame error
         return AVERROR_UNKNOWN;
 
-    avpkt->duration = ff_samples_to_time_base(avctx, frame->nb_samples);
     if (frame) {
+        avpkt->duration = ff_samples_to_time_base(avctx, frame->nb_samples);
         if (frame->pts != AV_NOPTS_VALUE)
             avpkt->pts = frame->pts - ff_samples_to_time_base(avctx, avctx->initial_padding);
     } else {