negative key for a key action will now be bound to unknown scancode

src/input.c

@@ -88,7 +88,7 @@ void input_add_axis_action(SDL_Scancode negative, SDL_Scancode positive, InputAc
 void input_add_key_action(SDL_Scancode key, InputActionDelegate delegate) {
     _resize_actions_if_needed(_actions_len + 1);
     _actions[_actions_len] = (InputAction) {
-        .negative = NULL,
+        .negative = _keys + SDL_SCANCODE_UNKNOWN,
         .positive = _keys + key,
         .delegate = delegate
     };