Fix unmapped branch stairs on premapped levels

Fix unmapped branch stairs on premapped levels

This happens when levelporting to the first Sokoban level in wizard mode
before visiting the level, causing the branch stairs to not appear until
the space it is in comes in sight of the player.

The issue was that levels flagged premapped would cause the special
level coder to call sokoban_detect() before fixup_special() had a chance
to place the branch stairs properly.

Fix from Dynahack by Tung Nguyen.