squash4: Fix an uninitialized variable

gcc says:

grub-core/fs/squash4.c: In function ‘direct_read’:
grub-core/fs/squash4.c:868:10: error: ‘err’ may be used uninitialized in
this function [-Werror=maybe-uninitialized]
  868 |       if (err)
      |          ^
cc1: all warnings being treated as errors

This patch initializes it to GRUB_ERR_NONE.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/squash4.c

@@ -746,7 +746,7 @@ direct_read (struct grub_squash_data *data,
 	     struct grub_squash_cache_inode *ino,
 	     grub_off_t off, char *buf, grub_size_t len)
 {
-  grub_err_t err;
+  grub_err_t err = GRUB_ERR_NONE;
   grub_off_t cumulated_uncompressed_size = 0;
   grub_uint64_t a = 0;
   grub_size_t i;