Observed levels in Yeek's den (most frequently level 12) with patches of unrecognied...

Observed levels in Yeek's den (most frequently level 12) with patches of unrecognied terrain and had crashes on such levels where the common symptom was an out-of-bounds feature index.  Changed floor-generate.c so that nothing is done when adding a river to a dungeon, like Yeek's den, that does not accept any of the river types.  Have not seen levels with patches of unrecognized terrain since the change.