rtsp: fix live stream duration reporting

Bug: 28234978
Change-Id: If5be85cd92f4da2b711dc9409abd812ebf910a64
diff --git a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
index c4e5df7..7ce909d 100644
--- a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
@@ -258,7 +258,7 @@
 }
 
 status_t NuPlayer::RTSPSource::getDuration(int64_t *durationUs) {
-    *durationUs = 0ll;
+    *durationUs = -1ll;
 
     int64_t audioDurationUs;
     if (mAudioTrack != NULL