Fix a regression caused by "efi: fix some malformed device path arithmetic errors"

This commit introduced a bogus check inside copy_file_path to
determine whether the destination grub_efi_file_path_device_path_t
was valid before anything was copied to it. Depending on the
contents of the heap buffer, this check could fail which would
result in copy_file_path returning early.

Without any error propagated to the caller, make_file_path would
then try to advance the invalid device path node with
GRUB_EFI_NEXT_DEVICE_PATH, which would also fail, returning a NULL
pointer that would subsequently be dereferenced.

Remove the bogus check, and also propagate errors from copy_file_path.

Patch-Name: efi-malformed-device-path-2.patch