fix EXT-4858 "Detecting Hardware..." not showing correctly in non-English languages with Viewer 2.0

reviewed Vadim Savchuk
https://codereview.productengine.com/secondlife/r/317/

--HG--
branch : product-engine

indra/llwindow/llwindowwin32.cpp

@@ -2877,8 +2877,13 @@ void LLSplashScreenWin32::updateImpl(const std::string& mesg)
 {
 	if (!mWindow) return;
 
+	int output_str_len = MultiByteToWideChar(CP_UTF8, 0, mesg.c_str(), mesg.length(), NULL, 0);
+	if( output_str_len>1024 )
+		return;
+
 	WCHAR w_mesg[1024];
-	mbstowcs(w_mesg, mesg.c_str(), 1024);
+
+	MultiByteToWideChar (CP_UTF8, 0, mesg.c_str(), mesg.length(), w_mesg, output_str_len);
 
 	SendDlgItemMessage(mWindow,
 		666,		// HACK: text id