[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>