fix: change list length in data_structures/list/list.c (#1265)

I changed the return value of n in List_length to reflect the number of items inside the list, so a newly initialized list will return a length of 0.  To prevent items in List_toArray from being cut off, I addeone back to n at the beginning of the List_toArray function.

data_structures/list/list.c

@@ -30,13 +30,13 @@ int List_length(L list)
 {
     int n;
     for (n = 0; list; list = list->next) n++;
-    return n;
+    return n - 1;
 }
 
 /* Convert list to array */
 void **List_toArray(L list)
 {
-    int i, n = List_length(list);
+    int i, n = List_length(list) + 1;
     void **array = (void **)malloc((n + 1) * sizeof(*array));
 
     for (i = 0; i < n; i++)