USB: cdc-acm: fix potential null-pointer dereference on disconnect

USB: cdc-acm: fix potential null-pointer dereference on disconnect

Fix potential null-pointer exception on disconnect introduced by commit
11ea859d64b69a747d6b060b9ed1520eab1161fe (USB: additional power savings
for cdc-acm devices that support remote wakeup).

Only access acm->dev after making sure it is non-null in control urb
completion handler.

Cc: stable <stable@kernel.org>;
Signed-off-by: Johan Hovold <jhovold@gmail.com>;
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>;

BUG=chrome-os-partner:3143
TEST=run power_Resume test in tight loops on Alex.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>;

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=bdfbe25

[Vincent Palatin, 2011-04-11 17:01:42]

[3/5] try to fix the issues detected in the CDC ACM driver when running
power_Resume test with YT3300 modem inside.

it's the backport of an upstream patch

[Vincent Palatin, 2011-04-11 17:03:14]

fixing Sonny's address

[Sonny, 2011-04-12 00:21:12]

LGTM