fix: $79 boundary check (#398)

修改
        if (board == null || word == null) return false;
        if (word.length() == 0) return true;
        if (board.length == 0) return false;

Author: ruyadebaozi

problems/79.word-search.md

@@ -136,8 +136,9 @@ _Java Code_
 ```java
 public class LC79WordSearch {
   public boolean exist(char[][] board, String word) {
-    if (board == null || board.length == 0 || board[0].length == 0
-        || word == null || word.length() == 0) return true;
+    if (board == null || word == null) return false;
+    if (word.length() == 0) return true;
+    if (board.length == 0) return false;
     int rows = board.length;
     int cols = board[0].length;
     for (int r = 0; r < rows; r++) {