sdl2: create_tech_tree() trouble if reqs not filled from beginning
sdl2-client create_tech_tree() assumes that *first* req_count requirement_buttons are non-NULL. It will crash, e.g., if there's one requirement but in second slot.
Patches for all branches, including S2_6 one handling also sdl-client, attached.
sdl2-client create_tech_tree() assumes that *first* req_count requirement_buttons are non-NULL. It will crash, e.g., if there's one requirement but in second slot.