fs/tar: Initialize name in grub_cpio_find_file()

fs/tar: Initialize name in grub_cpio_find_file()

It was possible to iterate through grub_cpio_find_file() without
allocating name and not setting mode to GRUB_ARCHELP_ATTR_END, which
would cause the uninitialized value for name to be used as an argument
for canonicalize() in grub_archelp_dir().

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>