fix memory corruption in pubkey filter over network

fix memory corruption in pubkey filter over network

grub_pubkey_open closed original file after it was read; it set
io->device to NULL to prevent grub_file_close from trying to close device.
But network device itself is stacked (net -> bufio); and bufio preserved
original netfs file which hold reference to device. grub_file_close(io)
called grub_bufio_close which called grub_file_close for original file.
grub_file_close(netfs-file) now also called grub_device_close which
freed file->device->net. So file structure returned by grub_pubkey_open
now had device->net pointed to freed memory. When later file was closed,
it was attempted to be freed again.

Change grub_pubkey_open to behave like other filters - preserve original
parent file and pass grub_file_close down to parent. In this way only the
original file will close device. We really need to move this logic into
core instead.

Also plug memory leaks in error paths on the way.

Reported-By: Robert Kliewer <robert.kliewer@gmail.com>
Closes: bug #43601