Document AudioSystem::newAudioSessionId() failures

Change-Id: Iaa168722f362c36bdfa87fe20dc0a59b43cf1ca3
diff --git a/media/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index 140fb66..c418466 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -431,7 +431,7 @@
 int AudioSystem::newAudioSessionId()
 {
     const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
-    if (af == 0) return 0;
+    if (af == 0) return AUDIO_SESSION_ALLOCATE;
     return af->newAudioSessionId();
 }