fixed bug that cause invaders sample to be unable to run

So if it had worked on VS2012, my guess is that is problem is specific to VS2013.

When running the "invaders" example in debug mode I saw this error: 

"Unhandled exception at 0x00BAAF84 in invaders.exe: 0xC000041D: An unhandled exception was encountered during a user callback."

which occured at Samples/shell/src/win32/ShellWin32.cpp at line 283:

    shell_renderer->SetViewport(width, height);

called from line 141:

    SetWindowPos(window_handle, HWND_TOP, 0, 0, window_rect.right - window_rect.left, window_rect.bottom - window_rect.top, SWP_NOACTIVATE);

So it turned out shell_renderer had not be initialized and was therefore NULL when called at that point. I then figured that the code block:

	if (_shell_renderer != NULL)
	{
		shell_renderer = _shell_renderer;
		if(!shell_renderer->AttachToNative(window_handle))
		{
			CloseWindow();
			return false;
		}
	}

initializes shell_renderer. Placing it before line 141 therefore solves this problem.

VS2013-32bit, Windows 8.1