Fix app crash when clicking spaces after slide page items in presentation demo.

[Zhwt]

The SetHighlightedFunc function is missing boundary check, so when clicking outside of available item list, it will panic with out of bound error.

tview/demos/presentation/main.go

Lines 57 to 59 in c766eef

	SetHighlightedFunc(func(added, removed, remaining []string) {
	pages.SwitchToPage(added[0])
	})

Fixed by checking slice length first.