Fix bug in largest_square: y direction

We were not taking into account that the largest_square may be
constrained by the y position as well as the x.  i.e. is there enough
vertical space for the square.

This commit adds that check.

We were working before because C++ is not memory safe, and we weren't
running debug builds.

main.cc

@@ -137,7 +137,9 @@ struct Grid {
       if (grid_[b + y(pos) * length_] != '.') { break; }
       ++b;
     }
-    return b - x(pos);
+    auto len = b - x(pos);
+    auto ye = std::min(y(pos) + len, length_);
+    return ye - y(pos);
   }
 
   /** Get the next position to check.  n is the size of the square we just