MAINT-5232: Convert gMessageSystem != NULL to simple bool test.

Now that gMessageSystem is an LLPounceable, we would either have to define
comparisons to LLPounceable's held type or static_cast<LLMessageSystem*> to
literally compare to NULL. But since we already define operator bool(), that's
the easy (and idiomatic) fix.

indra/newview/llmutelist.cpp

@@ -152,7 +152,7 @@ LLMuteList* LLMuteList::getInstance()
 {
 	// Register callbacks at the first time that we find that the message system has been created.
 	static BOOL registered = FALSE;
-	if( !registered && gMessageSystem != NULL)
+	if( !registered && gMessageSystem)
 	{
 		registered = TRUE;
 		// Register our various callbacks