Don't rely on argv being NULL-terminated under MSW

Since the changes of c9a458bfe8 ("Use Win32
::CommandLineToArgvW() to tokenize command line"), this is not guaranteed any
more as this Win32 function doesn't necessarily ensure it under older MSW
versions such as XP.

Just use "argc" explicitly instead of relying on this in wxCmdLineArgsArray to
fix crashing under XP.

src/gtk/app.cpp

@@ -396,7 +396,7 @@ bool wxApp::Initialize(int& argc_, wxChar **argv_)
 
     // update internal arg[cv] as GTK+ may have removed processed options:
     this->argc = argc_;
-    this->argv = argv_;
+    this->argv.Init(argc_, argv_);
 
     if ( m_traits )
     {