Commit cb2b844 and subsequent commits changed load_exit_models to only
call bm_free_extra_objbitmaps if EMULATING_D1. The variable
extra_bitmap_num is initialized as a side effect of
bm_free_extra_objbitmaps, so it wasn't initialized anymore if
not EMULATING_D1. This broke the exit sequence with the
D2 Mac Demo data and add-on missions with custom exit sequences.
This commit adds initialization of extra_bitmap_num if not EMULATING_D1
to fix the exit handling.
1d55bc7746