src/terminal/tab.c (get_tab_by_number): Assert that the returned
struct window * actually points to a struct window.
src/terminal/tab.c (move_current_tab): Keep tabs contiguous in the
stack of windows. Obey "ui.tabs.wraparound".
src/terminal/terminal.h (struct terminal): Documented that tabs can
move in the stack.
src/terminal/window.c (assert_window_stacking): Check that the main
menu isn't between tabs.