fix mismatched subscript

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2564 35c25a1d-7b9e-4130-9fde-d3aeb78583b8

src/files.c

@@ -1275,7 +1275,7 @@ void init_backup_dir(void)
  * write error. */
 int copy_file(FILE *inn, FILE *out)
 {
-    char buf[MAX_BUF_SIZE];
+    char buf[BUFSIZ];
     size_t charsread;
     int retval = 0;