ff_samples_to_time_base: support AV_NOPTS_VALUE

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

libavcodec/internal.h

@@ -153,6 +153,8 @@ int ff_alloc_packet(AVPacket *avpkt, int size);
 static av_always_inline int64_t ff_samples_to_time_base(AVCodecContext *avctx,
                                                         int64_t samples)
 {
+    if(samples == AV_NOPTS_VALUE)
+        return AV_NOPTS_VALUE;
     return av_rescale_q(samples, (AVRational){ 1, avctx->sample_rate },
                         avctx->time_base);
 }