Commit e6875641c9 moved allowed_chars from global scope into
individual newmenu_item entries. However, it did not enforce that this
field be initialized, and some callers failed to do so. The save game
menu was such a caller, and crashed when using an uninitialized value as
the allowed_chars pointer. There is no character restriction here, so
explicitly set the pointer to nullptr.
Reported-by: kitelessd <https://github.com/dxx-rebirth/dxx-rebirth/issues/571>
Fixes: e6875641c9 ("Move Newmenu_allowed_chars into individual newmenu_item")