[PATCH] uml: fix device unplug crash

Fix a NULL dereference when unplugging a device.  The default value of
err_msg wants to be "" in case the driver doesn't modify it.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/mconsole_kern.c

@@ -615,6 +615,9 @@ void mconsole_remove(struct mc_request *req)
 	err_msg = NULL;
 	err = (*dev->remove)(n, &err_msg);
 	switch(err){
+	case 0:
+		err_msg = "";
+		break;
 	case -ENODEV:
 		if(err_msg == NULL)
 			err_msg = "Device doesn't exist";