[issue251] Panic on -current

[walt]

Matthew Dillon wrote:
>     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).

Yes, there is a USB scanner plugged in, and I never unplugged it.

What is a VCHR device?