[issue251] Panic on -current

[Matthew Dillon]

    It crashed on a NULL pointer indirection trying to mmap() ucanner0.

    Do you have a USB scanner connected?  I can do a NULL check easily
    enough but it should not have been able to open a descriptor in the
    first place if there was no scanner.  Or, if there was a scanner, 
    maybe it got unplugged at some point before the mmap (though that
    shouldn't have NULLed out the field either).

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>