Null protect invocation of getContentResolver.

Bug: 22118779
Change-Id: I9a11c2fc05f8a6f418bc0e6daa55dfb61707bd6a
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index d0fe2e4..92bc089 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -339,7 +339,7 @@
     }
 
     private boolean queryHasContent(Uri voicemailUri) {
-        if (voicemailUri == null) {
+        if (voicemailUri == null || mContext == null) {
             return false;
         }