Filling: fix assertion failure in 3x1 game generation.

This would come up on the game id "3x1#12345", for example. The
failing assertion was (s->board[f] != EMPTY) in expand(), called in
turn from learn_expand_or_one().

It looks as if the problem was that the #define SENTINEL was set too
small. It was intended to be a value that can't coincide with the true
size of any region - and it was set to precisely the area of the whole
board. But on a 3x1 grid, that _can_ coincide with the size of a
region! So a board entry was set to a real region size, and then
mistaken for SENTINEL by another part of the code.

Easy fix: set SENTINEL to be sz+1. Now it really can't coincide with a
region area.