Don't allocate a 512 byte buffer on the stack in snplwrite. It's

Don't allocate a 512 byte buffer on the stack in snplwrite.  It's
probably harmless in this case, since the latter is called on tty
input, which is usually a result of some system call, so we've got
plenty of stack left.  It's still nice to fix these things, though, in
case somebody ever decides this driver is a good example of something
(perhaps "what you probably shouldn't do").