devres: WARN() and return, don't crash on device_del() of uninitialized device

I just debugged an obscure crash caused by a device_del() of a all NULL'd
out struct device (in usb-serial) and found that a patch like this one would
have saved me time (in addition to improved chances of a bug report from
users hitting similar driver bugs).

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>