BFU: Fix infinite loops in menus that have only unselectable elements.

BFU: Fix infinite loops in menus that have only unselectable elements.

src/bfu/menu.c (scroll_menu): Let neither menu->selected nor pos
become -2.

src/bfu/menu.c (menu_mouse_handler): Call set_menu_selection directly
rather than via scroll_menu, as sel is already known to be selectable.
(Not required for fixing the bug.)

src/bfu/menu.c (menu_search_handler): Break infinite loops also if
menu->selected is -1 initially.

src/bfu/menu.c (menu_handler): Instead of tweaking menu->selected
directly, let scroll_menu do it.