[issue251] Panic on -current

[Matthew Dillon]

    I think the descriptor might have gotten revoked with revoke().  Since
    it is a VCHR device, that would have succeeded.  But mmap() doesn't
    have the correct check for a revoked device descriptor so I will go
    ahead and add the check.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>