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 <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/drivers/line.c

···
 			parse_chan_pair(NULL, line, n, opts, error_out);
 			err = 0;
 		}
+		*error_out = "configured as 'none'";
 	} else {
 		char *new = kstrdup(init, GFP_KERNEL);
 		if (!new) {
···
 			}
 		}
 		if (err) {
+			*error_out = "failed to parse channel pair";
 			line->init_str = NULL;
 			line->valid = 0;
 			kfree(new);