Bug 1367376: Check for null. r=baku

MozReview-Commit-ID: LjLW4VJk4vh

--HG--
extra : rebase_source : e12b70d9974436580af926f636757856009ad5d4

dom/media/webspeech/synth/nsSpeechTask.cpp

@@ -783,7 +783,7 @@ nsSpeechTask::SetAudioOutputVolume(float aVolume)
   if (mStream && !mStream->IsDestroyed()) {
     mStream->SetAudioOutputVolume(this, aVolume);
   }
-  if (mIndirectAudio) {
+  if (mIndirectAudio && mCallback) {
     mCallback->OnVolumeChanged(aVolume);
   }
 }