USB: cdc-acm: Fixing crash when ACM probing interfaces with no endpoint descriptors.

USB: cdc-acm: Fixing crash when ACM probing interfaces with no endpoint descriptors.

Certain USB devices, such as the Nokia X6 mobile phone, don't expose any
endpoint descriptors on some of their interfaces. If the ACM driver is forced
to probe all interfaces on a device the a NULL pointer dereference will occur
when the ACM driver attempts to use the endpoint of the alternative settings.
One way to get the ACM driver to probe all the interfaces is by using the
/sys/bus/usb/drivers/cdc_acm/new_id interface.

This patch checks that the endpoint pointer for the current alternate settings
is non-NULL before using it.

Signed-off-by: Toby Gray <toby.gray@realvnc.com>
Cc: Oliver Neukum <oliver@neukum.name>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>