Bug 1394024 - Handle null string when alerting plugin about Windows microphone change. r=jimm

Fixes crash when unplugging the only available microphone.

--HG--
extra : rebase_source : 89740b96576e912b42efe3465961452d39a16433

dom/plugins/ipc/PluginUtilsWin.cpp

@@ -86,7 +86,7 @@ public:
     NPAudioDeviceChangeDetailsIPC changeDetails;
     changeDetails.flow = (int32_t)flow;
     changeDetails.role = (int32_t)role;
-    changeDetails.defaultDevice = std::wstring(device_id);
+    changeDetails.defaultDevice = device_id ? std::wstring(device_id) : L"";
 
     // Make sure that plugin is notified on the main thread.
     RefPtr<AudioDeviceChangedRunnable> runnable =