SL-19204 Crash at removeObserver

A lot of reports about crashes in voice's removeObserver with no indication of the cause, all MAC specific. By this point terminate should have been called and mVoiceModule should be null, yet callstaks suggest it isn't.

Commit clears LLVoiceClient beforehand to avoid the issue entirely, but issue lies elsewhere and 'voice' crash is just a symptom.

indra/newview/llappviewer.cpp

@@ -2164,6 +2164,7 @@ bool LLAppViewer::cleanup()
 	LLSelectMgr::deleteSingleton();
 	LLViewerEventRecorder::deleteSingleton();
     LLWorld::deleteSingleton();
+    LLVoiceClient::deleteSingleton();
 
 	// It's not at first obvious where, in this long sequence, a generic cleanup
 	// call OUGHT to go. So let's say this: as we migrate cleanup from

indra/newview/llpanelpeople.cpp

@@ -346,7 +346,10 @@ public:
 	{
 		// will be deleted by ~LLInventoryModel
 		//delete mInvObserver;
-		LLVoiceClient::getInstance()->removeObserver(this);
+        if (LLVoiceClient::instanceExists())
+        {
+            LLVoiceClient::getInstance()->removeObserver(this);
+        }
 		LLAvatarTracker::instance().removeObserver(this);
 	}
 

indra/newview/llvoiceclient.cpp

@@ -142,6 +142,7 @@ LLVoiceClient::LLVoiceClient(LLPumpIO *pump)
 
 LLVoiceClient::~LLVoiceClient()
 {
+    llassert(!mVoiceModule);
 }
 
 void LLVoiceClient::init(LLPumpIO *pump)