Fix integer underflow panic in game of life wrap around example

examples/nature_of_code/chp_07_cellular_automata/7_game_of_life_wrap_around.rs

@@ -60,10 +60,13 @@ impl Gol {
             for y in 0..self.rows {
                 // Add up all the states in a 3x3 surrounding grid
                 let mut neighbors = 0;
-                for i in 0..3 {
-                    for j in 0..3 {
-                        neighbors += self.board[(x + (i - 1) + self.columns) % self.columns]
-                            [(y + (j - 1) + self.rows) % self.rows];
+                for i in 0..3i32 {
+                    for j in 0..3i32 {
+                        let cols = self.columns as i32;
+                        let rows = self.rows as i32;
+                        let board_x = (x as i32 + (i - 1) + cols) % cols;
+                        let board_y = (y as i32 + (j - 1) + rows) % rows;
+                        neighbors += self.board[board_x as usize][board_y as usize];
                     }
                 }