audio dec: set timestamp from first packet

src/audio/audio.cpp

@@ -782,6 +782,7 @@ static void *audio_receiver_thread(void *arg)
                                 struct audio_decoder *dec_state = (struct audio_decoder *) cp->decoder_state;
                                 if (dec_state && dec_state->enabled) {
                                         dec_state->pbuf_data.buffer.data_len = 0;
+                                        dec_state->pbuf_data.buffer.timestamp = -1;
                                         // We iterate in loop since there can be more than one frmae present in
                                         // the playout buffer and it would be discarded by following pbuf_remove()
                                         // call.

src/rtp/audio_decoders.cpp

@@ -842,7 +842,9 @@ int decode_audio_frame(struct coded_data *cdata, void *pbuf_data, struct pbuf_st
                 }
         }
         s->buffer.data_len = new_data_len;
-        s->buffer.timestamp = decompressed.get_timestamp();
+        if (s->buffer.timestamp == -1) {
+                s->buffer.timestamp = decompressed.get_timestamp();
+        }
 
         decoder->decoded.append(decompressed);