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unix: fix the ad hoc STAILQ_PREPEND()

Description

unix: fix the ad hoc STAILQ_PREPEND()

If there is nothing to prepend, don't try STAILQ_INSERT_HEAD().

Fixes: d80a97def9a1db6f07f5d2e68f7ad62b27918947
Reported-by: syzbot+bb7f3d07c79b5faf8de8@syzkaller.appspotmail.com