um: line: always fill *error_out in setup_one_line()

The pointer isn't initialized by callers, but I have
encountered cases where it's still printed; initialize
it in all possible cases in setup_one_line().

Link: https://patch.msgid.link/20240703172235.ad863568b55f.Iaa1eba4db8265d7715ba71d5f6bb8c7ff63d27e9@changeid


Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>