sparc: Fix cleanup crash in bbc_envctrl_cleanup()

If kthread_run() fails or never gets to run we'll have NULL
or a pointer encoded error in kenvctrld_task, rather than
a legitimate task pointer.

So this makes bbc_envctrl_cleanup() crash as it passed this
bogus pointer into kthread_stop().
Reported-by: BERTRAND Joël <joel.bertrand@systella.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/sbus/char/bbc_envctrl.c

@@ -537,8 +537,12 @@ int bbc_envctrl_init(struct bbc_i2c_bus *bp)
 	}
 	if (temp_index != 0 && fan_index != 0) {
 		kenvctrld_task = kthread_run(kenvctrld, NULL, "kenvctrld");
-		if (IS_ERR(kenvctrld_task))
-			return PTR_ERR(kenvctrld_task);
+		if (IS_ERR(kenvctrld_task)) {
+			int err = PTR_ERR(kenvctrld_task);
+
+			kenvctrld_task = NULL;
+			return err;
+		}
 	}
 
 	return 0;
@@ -561,6 +565,7 @@ void bbc_envctrl_cleanup(struct bbc_i2c_bus *bp)
 	struct bbc_cpu_temperature *tp, *tpos;
 	struct bbc_fan_control *fp, *fpos;
 
+	if (kenvctrld_task)
 		kthread_stop(kenvctrld_task);
 
 	list_for_each_entry_safe(tp, tpos, &bp->temps, bp_list) {