lavd/pulse_audio_enc: add pointer checks

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>

libavdevice/pulse_audio_enc.c

@@ -692,7 +692,9 @@ static void pulse_get_output_timestamp(AVFormatContext *h, int stream, int64_t *
     pa_threaded_mainloop_lock(s->mainloop);
     pa_stream_get_latency(s->stream, &latency, &neg);
     pa_threaded_mainloop_unlock(s->mainloop);
+    if (wall)
         *wall = av_gettime();
+    if (dts)
         *dts = s->timestamp - (neg ? -latency : latency);
 }