When a usb mass storage device is removed the related CAM SIM structure is

When a usb mass storage device is removed the related CAM SIM structure is
terminated.  A ref count prevents the actual sim structure from being
destroyed if the device still has refs, but the sim->devq structure is not so
protected and will be NULLed out.  An unmount attempt after device removal
will then result in a panic.

Fix this by testing for a NULL sim->devq.

Reported-by: Eduardo Tongson <propolice@gmail.com>