[TD-3783]<fix>: taosdemo for windows generates rand string invalid. (#5828)

src/kit/taosdemo/taosdemo.c

@@ -1063,7 +1063,7 @@ static void rand_string(char *str, int size) {
     //--size;
     int n;
     for (n = 0; n < size - 1; n++) {
-      int key = rand_tinyint() % (int)(sizeof(charset) - 1);
+      int key = abs(rand_tinyint()) % (int)(sizeof(charset) - 1);
       str[n] = charset[key];
     }
     str[n] = 0;