[Chromecast] Fix crash when ALSA volume element has no mixer

Bug: internal b/74200001
Change-Id: I44c5cfacd53acea6188f5c9753907d8f2d995763
Reviewed-on: https://chromium-review.googlesource.com/950141Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541052}

chromecast/media/cma/backend/alsa/alsa_volume_control.cc

@@ -349,11 +349,13 @@ void AlsaVolumeControl::RefreshMixerFds(ScopedAlsaMixer* mixer) {
 }
 
 void AlsaVolumeControl::OnFileCanReadWithoutBlocking(int fd) {
-  alsa_->MixerHandleEvents(volume_mixer_->mixer);
-  if (mute_mixer_) {
+  if (volume_mixer_->mixer) {
+    alsa_->MixerHandleEvents(volume_mixer_->mixer);
+  }
+  if (mute_mixer_ && mute_mixer_->mixer) {
     alsa_->MixerHandleEvents(mute_mixer_->mixer);
   }
-  if (amp_mixer_) {
+  if (amp_mixer_ && amp_mixer_->mixer) {
     // amixer locks up if we don't call this for unknown reasons.
     alsa_->MixerHandleEvents(amp_mixer_->mixer);
   }