[3.x] Fix PCKPacker error spam

[dzil123]

After PCKPacker.flush(), the file is closed and it is invalid to call flush() or add_file(), but the closed file isn't properly detected because the FileAccess remains nonnull and add_file() doesn't check for a closed file, causing flush() to spam error messages and add_file() to miss the error. This PR fixes that.

With this script:

extends Node

func _ready():
        var packer = PCKPacker.new()
        packer.pck_start("res://out.pck")
        packer.flush()
        packer.flush()
        print()

        packer.add_file("", "icon.png")

Error messages before:

ERROR: File must be opened before use.
   at: store_8 (drivers/unix/file_access_unix.cpp:261)
ERROR: File must be opened before use.
   at: store_8 (drivers/unix/file_access_unix.cpp:261)
ERROR: File must be opened before use.
   at: store_8 (drivers/unix/file_access_unix.cpp:261)
ERROR: File must be opened before use.
   at: store_8 (drivers/unix/file_access_unix.cpp:261)
ERROR: File must be opened before use.
   at: get_position (drivers/unix/file_access_unix.cpp:205)
ERROR: File must be opened before use.
   at: get_position (drivers/unix/file_access_unix.cpp:205)

Error messages after:

ERROR: File must be opened before use.
   at: flush (core/io/pck_packer.cpp:111)

ERROR: File must be opened before use.
   at: add_file (core/io/pck_packer.cpp:89)

Compatible with 3.5.

[akien-mga]

Is this relevant for 4.0?

[dzil123]

The equivalent 4.0 PR is #67619.

[akien-mga]

Thanks!

[timothyqiu]

Cherry-picked for 3.5.2