Fix incorrect ISO metadata being written after shrinking

[Enzime]

When I try to add a new file into a Windows 11 ISO using the following script:

import pycdlib

iso = pycdlib.PyCdlib()
iso.open('/Users/enzime/Downloads/26100.4349.250607-1500.ge_release_svc_refresh_CLIENTCONSUMER_RET_A64FRE_en-us.iso')
iso.add_file('/Users/enzime/Downloads/autounattend.xml', udf_path='/autounattend.xml')
iso.write('/Users/enzime/Downloads/Win11_unattended.iso')
iso.close()

iso = pycdlib.PyCdlib()
iso.open('/Users/enzime/Downloads/Win11_unattended.iso')

# Check UDF root for autounattend.xml
for child in iso.list_children(udf_path='/'):
    if child is None:
        continue
    if child.is_dot() or child.is_dotdot():
        continue
    name = child.file_identifier().decode('utf-8', errors='replace')
    print(name)

iso.close()

print('Done')

I would get the following error:

Traceback (most recent call last):
  File "<stdin>", line 10, in <module>
  File "/nix/store/187j0l8n5f1vwh85pq8fc78z5jqqf9ha-python3-3.13.11-env/lib/python3.13/site-packages/pycdlib/pycdlib.py", line 4066, in open
    self._open_fp(fp)
    ~~~~~~~~~~~~~^^^^
  File "/nix/store/187j0l8n5f1vwh85pq8fc78z5jqqf9ha-python3-3.13.11-env/lib/python3.13/site-packages/pycdlib/pycdlib.py", line 2319, in _open_fp
    self._parse_udf_descriptors()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/nix/store/187j0l8n5f1vwh85pq8fc78z5jqqf9ha-python3-3.13.11-env/lib/python3.13/site-packages/pycdlib/pycdlib.py", line 2017, in _parse_udf_descriptors
    raise pycdlibexception.PyCdlibInvalidISO('Expected at least 2 UDF Anchors')
pycdlib.pycdlibexception.PyCdlibInvalidISO: Expected at least 2 UDF Anchors

With this PR, the output ISO can be successfully opened and read:

boot
boot.catalog
bootmgr.efi
efi
setup.exe
sources
support
autounattend.xml
Done