maybe_backup_file: Clear previously-set after_backup_name

maybe_backup_file: Clear previously-set after_backup_name

Without this, under various conditions, (including file_name being
a directory), maybe_backup_file would return true without having
done anything, nor having set after_backup_name. Then, if any
error was detected while extracting the member, (such as directory
already exists), the previously backed-up file would be restored,
potentially destroying user data.

This closes Debian bug #508199, with thanks to Eric Lammerts for
reporting it with a concise test case.