Trivial initialization fix in AudioDeviceIOS

NOTRY=TRUE
TBR=tkchin
BUG=webrtc:5058

Review URL: https://codereview.webrtc.org/1435003002

Cr-Commit-Position: refs/heads/master@{#10616}
diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.mm b/webrtc/modules/audio_device/ios/audio_device_ios.mm
index 5bf6b3b..1466d6a 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.mm
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.mm
@@ -198,7 +198,8 @@
       initialized_(false),
       rec_is_initialized_(false),
       play_is_initialized_(false),
-      audio_interruption_observer_(nullptr) {
+      audio_interruption_observer_(nullptr),
+      route_change_observer_(nullptr) {
   LOGI() << "ctor" << ios::GetCurrentThreadDescription();
 }