Don't call isPlaying if not prepared.

Bug: 21647046
Change-Id: Ib0f5c2d7cd973ba4732defeceb32fe956e8dbdb8
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 99d2734..8db7852 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -204,7 +204,7 @@
 
     public void onPause(boolean isFinishing) {
         // Do not pause for orientation changes.
-        if (mMediaPlayer.isPlaying() && isFinishing) {
+        if (mIsPrepared && mMediaPlayer.isPlaying() && isFinishing) {
             pausePlayback();
         }