Local map rendering in some cells is broken

Steps to reproduce:

1. coc "Nchuleftingth, Test of Pattern"
2. Use the third switch to open a hidden passage
3. Save the game
4. Load new savegame
5. Map looks like this for me (on both Windows and Linux):

There is a warning in the game log when loading the save:

Warning: expanding fog by 1, 0

This bug does not happen in 0.47, so label it as a regression so far.